Google will only send you a cheque (or bank transfer) when your AdSense account has $100 in it. RoughType describes how this may help Google a considerable amount. This has certainly annoyed some people.
But as the post says, the AdSense ToS states:
In the event the Agreement is terminated, Google shall pay Your earned balance to You within approximately ninety (90) days after the end of the calendar month in which the Agreement is terminated by You (following Google’s receipt of Your written request, including by email, to terminate the Agreement) or by Google. In no event, however, shall Google make payments for any earned balance less than $10.
Meaning that provide you have more than $10 in your account you can cancel it and Google will send you your cash. Then if you want you can apply again. Very useful, though I think I’ll wait for my $100.