Harrying is good. Darkness is good. And group tactics can be devastating.
First, vast numbers are very good for flanking in melee. If you can totally surround someone, they're completely flanked, and most of the goblins can start using Aid Another rolls (seven goblins aiding a single attacker gives +14 to hit, +16 when flanking is considered). A well-armed goblin with assistants can really deal out the damage, and really, swarming opponents is what goblins do best.
Second, grappling is a viable tactic. Goblins are small, so eight of them can grapple a single human, and once one of them has latched on, no more attacks of opportunity and no Dex bonus to AC. If you've got eight goblins hanging off you, there's a good chance one of them will actually beat you in a grapple check and pin you, dropping your AC 4 points for the surrounding goblins to jab at you. (Goblins are at a disadvantage when grappling powerful characters; their grapple bonus of BAB+STRmod+size mod is around -4, while 10th level rogues are likely to be closer to +7. Still, a difference of 11 can be beaten on a d20.)
(As a side note, I'd probably allow Aid Another actions to improve the grapple bonus. A tsunami of goblins should be able to bear even strong, experienced characters down. +14 to grapple? That's not too shabby!)
Third, goblins should be careful of opportunity attacks. That was the secondary reason the fighter in my campaign did so well against entire armies... an opportunity attack on a charging enemy, followed (in all likelihood) by a Cleave on the next one. (Then the Whirlwind Attack, but that's not too likely in this group.) If the goblins approach cautiously, they may take extra casualties from missile fire, but if any of the PCs have Combat Reflexes, it's likely that they'll be safer stopping 5' from the PCs and just taking a step on their next action. It might take longer to surround the PCs for flanking, too. I'd weigh it up depending on the tactical situation; if you have enough casters, Protection From Arrows (Sor/Wiz 2) can be very useful in this circumstance. (DR 10/+1 against all ranged weapons.)
Finally, if the PCs are holed up in a limited area, why bother firing 'filth'-tipped arrows when you could just fling buckets of 'filth' in the first place? I guarantee the PCs wouldn't have an easy time resting in those circumstances, and would almost certainly contract filth fever over the next couple of days. Slow Dex/Con drain ensues; after a week, they're down maybe 10 Dex, 10 Con and have very poor chances of ever shrugging off the disease. It's a nasty way to die. ("Eeww, it got into my iron rations!" Do we eat, or do we starve?)