When your adsense earnings reach a particular threshold (50 pounds, I think), Google sens a PIN through post to the address you gave in your google adsense account. This is to verity the address your payments will be sent to. Many people complain they can't get this PIN delivered and I was not an exception. This had my account on hold for a couple of months till a friend who had the same problem gave me a tip on what to do.
I wouldn't know if the PIN delivery issue was actually a problem with the Nigerian Postal Service or google itself, I was really worried and had to try other advertisers like bidvertiser when my sites started displaying public service ads from google. Mind you, public service ads wont fetch you a dime! I had to remove them and for a couple of months, I didn't know what to do and almost gave up on google adsense.