Originally Posted by
Kojak
I think Craig and his list are in your future. Not that there isn't anything available new for $400, but you'll probably get more value for your money if it's been ridden around the block a few times.
+1
With your budget you can do far better buying a good used bike than a crappy new bike (sadly, most new bikes below $400 are pretty crappy, with the exception of Bikesdirect.com and super simple fixies/cruisers).
If you are knowledgeable about bikes, you know what to look for. If not, there are plenty of folks around here that can give their (often diverse) opinions.
It will help to know your level of experience with bikes, frame sizing needs (inseam and height helps here), terrain and level of fitness.
Any bicycle can be used for the purpose of commuting, but some tend to offer functionality that most commuters appreciate having.