That's what I want to see the most in Smallville and the producers have mentioned they want it too. The thing is I think they only can do it if the Warner Bros. film division let's them (like with the Lois Lane character).
In the new graphic novel Superman/Batman: Public Enemies written by Jeph Loeb who is working in Smallville as well, there's a cool segment called "When Clark met Bruce" that would be cool to use with Clark and Pete playing baseball (or basketball in the show's case) and they see a sad kid near a limousine with a flat tire that Alfred is changing. Clark thinks of asking him to play with them but Pete says no, etc but Clark can't stop looking at him, etc
Just that cool cameo would be great, or to have them passing by at Lex's mansion where Lex and Bruce had a business meeting.
An adventure of both of them would be cool too but that would probably be too much.