I don't think I'd like to see
too many DC cameos on
Smallville, but a few would be nice... (I'm still waiting for a preteen Jimmy Olsen

)
Hal Jordan would be nice, but what about Abin Sur? He's Sector 2814's Green Lantern at the time, and in some Silver Age story I think it was told how he protected Kal-El's pod as it entered Earth's solar system. He's also an alien like Clark, so that could add some kind of dynamic to their interaction.
Even better than Abin Sur... Alan Scott? Or any Golden Ager, really. What if Clark and young Mister Allen have an encounter with Jay Garrick? I think the thing that keeps so many traditional super-heroes off the show is the fact that they wear colorful costumes, which apparently is a no-no as far as WB is concerned, but the original Flash didn't have that problem -- his uniform was just a shirt, jeans, boots, and that "Mercury" hat. And give the GA Green Lantern's costume slightly darker, earthier tones (blood red, burgundy, forest green), and suddenly it's not so gaudy anymore. Give Wildcat a leather jacket to wear with his mask and now he's not just some guy in tights. Know what I mean? Clark has to get the idea of wearing a costume to fight crime from somewhere.
And maybe, in the background, there could be hints that super-powered beings are making a comeback in the modern world, allowing for cameos from early Silver Age characters... Chloe might find a piece for the Wall of Weird regarding an alleged "Manhunter from Mars", or mutterings about "Captain Comet".
So... while I don't think a ton of guest appearances would be a good idea... a few more than we're getting now would be welcome.