1. Spellcasters start having their earth-based spells fail.
2. Armor starts increasing in weight and encumbrance (lodestone).
3. Thieves, hearing about these wondrous implements able to chisel through solid rock better than, well, chisels, start trying to steal them. (Prestidigitation, anyone?)
4. Heat Metal. As a random, daily-occurring curse, it can be truly inconvenient... say, at the Prince's Feast, where the PCs are the guests of honor. (You burned... down... my... PALACE!!!)
5. A group of Dwarves keep trying to buy the darn things back. They follow the PCs *EVERYWHERE*, even when they're trying to sneak (or dig, or spoon) their way into the BBEG's castle. They arrive inside to find a coalition of the Dwarven Historical Preservation and Recovery Society waiting for them, with a new and better offer. (Those priestly divination spells are pretty useful, y'know?) This, of course, has the side effect of alerting many of the PCs' opponents that a) they're coming, b) right now, and c) what they're armed with. Note that harming these Dwarves will bring down the wrath of all Dwarven nations - both on the PCs, and on the kingdom they're in. The repercussions should be harsh... and reputation-destroying.
6. The gold-hungry Dwarves are now cornering the metals markets, causing prices to skyrocket. The Dwarves are a) trying to buy the remains of the artifact back, b) trying to hire every mercenary/kingdom/party of adventurers they can to recover the ancient heirloom, c) hiring every guild of spellcasters (good, evil, lawful, chaotic - ALL of them) to locate the PCs - which should make life very interesting for them when these groups try to swipe the goods and sell them to the Dwarves, d) have gone to every Paladin on the continent with the problem and recruited their help, or e) any combination of the above.
7. Merchants don't want to deal with the PCs, but being merchants, want to make money. They know the PCs have freakin' *adamantine* spoons! They must be *loaded*! Triple all prices at anything town-sized or larger.
8. The hammer was actually the keystone item for keeping a demon Prince imprisoned. Now he's loose. And he doesn't want anyone putting the hammer back together to lock him up again. Best thing to do is throw all the pieces into a sphere of annihilation. And if the PCs won't cooperate, well, spheres of annihilation are good at disposing of all sorts of problems.
9. The Host of Heaven (Celestials) need adamantine to forge weapons for the Heaven vs. Hell war. If the PCs are good, they, of course, need to help. Kinda sucks when your clerics can't regain spells, and nobody can benefit from (good) healing magic anymore.
10. The Hordes of Hell know that their counterparts are buying up all the adamantine. So they curse and corrupt it, making it unusable for the good guys.
11. Buying up more adamantine trashes the market. Increase the price of adamantine by x10. x100 if the Dwarves are cornering the metals market. Of course, selling adamantine crashes the price in that area. This should lead lots of hordes of greedy BBEGs straight to the PCs.
12. Now the gods are irritated. PCs start receiving visions of this wronged, desecrated Dwarven hero. Spellcasters can't get their 8 hours of sleep at night. Healing/Curing spells start failing on PCs. Looks like an Atonement is in order.
13. Retrievers. One for each PC.
14. Bebiliths. One for each PC.
15. All the food the PCs try to eat is soaked with Dwarven blood. Vampires will love them.
16. PCs are now pursued by the vengeful spirit of the Dwarven hero. He inflicts ability score damage, and always shows up when they're in a fight against other creatures. Each time they defeat him, raise one of his classes by 5 levels, and he comes back 10 days later. In combination with any of the other items in this list, it's probably overkill.
17. The miners' guild refuses to work with them, after divinations reveal the PCs plans. Heck, the PCs can't even *find* mining equipment now.
18. The smiths' guild refuses to work with them, since the god of smiths is angered at the belittlement of the craft - hammering adamantine into spoons, indeed! Any smith who helps the PCs is cursed with a wasting dissease that causes all metals the smith touches to begin corroding... with a matching drop in the smiths' Str and Con.
19. Druids, alarmed at the potential rape of the land by the PCs plans, organize and ally against them. Being attacked by hordes of giant insects, plants, and animals - with the Dwarven nations helping - should result in either a quick parlay or a TPK.
20. Enforce the rules. A spoon is a poor choice for any sort of digging. Have the PCs spend *days* trying to chisel through manacles... with an unbreakable spoon. Each chiseling attempt is a to hit roll, and as has already been mentioned, would have penalties in combat. If it's being done in manacles, attach a -4 circumstance penalty. A spoon is, furthermore, an improvised weapon (more penalties). It is also *tiny* and awkward - no, you can't get a Str bonus using a spoon (maybe a larger weapon, such as a shovel, would work). So it would reasonably inflict 1hp, and threaten on a 20 (for x2). This is in addition to the penalties already mentioned. All the sudden, your halfling thief is trying to chisel his way out of manacles, making to hit rolls with around -12 to hit. Do the same with all the tunneling.
21. Dragons. Elementals (especially earth). Take your pick of any big, nasty, metal-greedy monster. Have it stomp a PC (or the whole party) and demand the adamantine to spare their lives. It also gives you a plothook for later when the PCs are higher level: Revenge is a great motivator.
22. Xorn/Xaren. At night. They'll eat the spoons and extra adamantine. Problem solved.
23. The Overprotective Forces of Good show up. The PCs have caused too much upheaval, and this must be corrected. The adamantine implements must be removed from these unwise/unknowing fools (or the PCs can fight a bunch of high-level clerics, paladins, rangers, and wizards, who will retreat before dying, and keep coming back) before several kingdoms economies collapse (or are laid waste by various parties' attempts to get the goods).
