I think this is a valid concern, and I'm glad I'm not the only one worried about it. There are quite a few calculators out there where you can get an idea of how different your take home pay will be. If absolutely nothing is done, it seems that my DH and I will be out almost $10K a year. That could be the extreme since we make very good money and don't have kids right now.
Chances are, something will be done with the fiscal cliff before the deadline (stupid broken Washington), and as long as you are not making $250-$300K a year, you should not feel a huge difference. However, I do believe that either way, the 2% payroll tax cut will end (I cannot see this being extended no matter what), so you will at least see a 2% tax increase.
If you are really that close to not being able to afford it, then maybe it would be best to wait and see. Always better to err on the side of caution than end up with a mortgage payment that you cannot happily afford (you should be able to have a life too, not be house poor).