If they do lose, there's still a 50/50 chance TCU (currently #18) would move into the top 16, which would give them the non-AQ bowl bid instead. As long as a conference champ from a non-BCS league is in the top 16 and ranked ahead of at least one of the BCS conference champs (BE), they have an auto bid. If Houston wins, that's a bid, if they lose, it's far from a given that a non-AQ gets one.
Under the scenario of Houston losing and TSU finishing outside the top 16, here's the rundown of the four at-large bids (assuming LSU beats Georgia, and Alabama remains ahead of OkSt in BCS):
1. Alabama
2. Stanford (locked into an at-large as long as they remain in the top 4 of the BCS, likely to Fiesta)
3. Big 12 at-large. Oklahoma/Oklahoma St/Kansas St. This will likely be the loser of the OSU/OU game this weekend. On the off chance Oklahoma fell to 15th with a bad loss (currently 10th), this could be Kansas State, although they'd be measured against other available options.
4. Here it gets interesting. The loser of the Big Ten CG is likely to fall out of the top 14 with the two teams at 13 and 15 respectively. Michigan is currently at 16. If neither the CG loser or Michigan cracks the top 14, the Big Ten will only have 1 bid. The only options for the at-large would be Boise St (likely #5 under this scenario) and VT. I think VT has a legitimate chance in that scenario.
That's asking a lot though, starting with Houston losing, so I'd call the scenario extremely unlikely.