I hate the idea of Tom going I really do. He is the conscious of that awful family and his scenes with Sybbie are delightful. So Julian Fellowes, don't be an ass and shoot yourself in the foot. Keep Tom around - but stop wasting his and the actor's potential with such awful story lines. They really suck!
Personally I think that Tom will return. I've seen two interviews where the moderator asked Allen Leech whether Branson will be part of season 6 and both times he was very vague and just said that "it would be too hard for Branson to stay away from Downton too long" and "he doesn't know whether people would like Branson to be back". Seriously...when the actor leaves the cast because he has other projects, he would just say it, wouldn't he?
PS: Sorry for my probably clumsy English (no native speaker ;-) ).
I think Allen Leech (Tom) will be around until the end of the Downton series though I am not sure how much and in what context. I just hope he doesn't come back from Boston as a failure - that would be awful to watch. I am not too bothered about a love interest either as I think it would be hard to find a worthy replacement for Sybil.
Is Tom leaving? Good question. As the character has been written, surely there is plenty of reason for him to feel wanted and that his work as Downton's agent is certainly important and must be done by someone.
Doesn't it seem odd, though, that Julian seems to be writing him out? He's been a major cog in the wheel since the beginning of the series. Unless production is thinking about having a reason to start an American storyline, and I really don't see that happening, what with the cost and all, then surely something will happen to make Tom want to stay.
At least that's what I'm hoping.