yes, what kuro said is true. while NNNNN.com can be zip codes, there isn't really any commercial value in it.
at least for nnnn.com, if you get a good combination like 2012.com, there is lots of commercial value since people offering products/services related to World Cup 2012 will have lots of interest in it.
nnnn.com is also shorter and only 10,000 of such are available compared to nnnnn.com of which 100,000 are available haha