I know this is a bit old, but since I went through this for 7 months before I figured it out, I figured I post it on here since I found this thread...
Go on your router and change the wifi encryption method. I forget which one, but like Android 2.0 and older, the nook does not work with all the popular standard encryption types, mine worked fine for 2 months before I had issues and would still work a few weeks sometimes before having problems, but my wife's nook would never get an IP from the router.
It is either the TKIP or AES setting, one works perfectly with the nook and the other does not play well. I spent $300 on Routers trying to find one that would work with my Nook, not realizing that the configuration I set up each time was causing the problem.