Tommy's been off the show since the beginning of last season (didn't you see the episode where the kids were searching for Leo, and found his cousin, Bobcat Goldthwaite, only to find he had left town?)
Tommy was on the John Boy and Billy Big Show last year, and they asked him about it. His response was to jokingly say "They started the season without me", and leave it at that.
Bummer dude. :) I watch the reruns in syndication. Haven't seen the new ones in a while.