According to sources at Kryptonsite rumors are J'onn J'onzz better known as the Martian Manhunter, will be featured in an upcoming episode of Smallville. Earlier fan speculation had a young Bruce Wayne making his debut on the show although those rumors were later debunked as it were.
The extra "cameo" appearance for the episode "Justice" is NOT Batman but it is...Martian Manhunter. you never see him but you do see a bag of half eaten oreos left beside clark when someone "saves" him.
i was at a comic shop near the studios that film smallville/stargate/battlestar galactica AND fantastic four 2 (its within 2-5 minutes of all these studios) here in vancouver and crew people who work on set regularly come by and drop tidbits of what they know. a crew member on smallville dropped that piece of news as he was filling his bag up with DC action figures.
"Justice", episode eleven of Smallville's season 6 which will also include appearances by Cyborg, Aquaman, Flash, and Green Arrow will be airing January of 07.