Remember, last year Tate was going to be THE GUY before he went down for the season, so I don't think Houston has no faith in the guy...Foster is a great back but he's also the product of a great system and a great O-line.
At this point, I'd say I can't trust Foster until he's absolutely 100% healthy and back to starting status. Even if he's healthy, Houston may roll with a time share (why wouldn't they when they've got two solid RB's?)...Either way I think it's safe to say Foster is a long shot to even come close to his '10 season.
I wouldn't try to trade Foster because he just has no value right now and there's no point, but I wouldn't try to trade Tate, because muscle strains can be problematic and linger, and if Foster keeps trying to come back and play when he's not fully healthy he's either going to re-injure himself or play like suck, so Tate has a great bit of upside right now.
I'd hold onto both, and start Tate in the short-term (at least while he's starting), and see how this whole thing plays out over the season.