You do have to be a pdga member at the time of the tournament, and also pass an officials exam, which is online and 'open book'.
If you play in any pdga sanctioned events this year
, you will earn points. Invites for 2012 will be based on those points. If you get invited, you can register before everyone else (around March) Then after a certain date (around May), anybody who wants to pay the entry fee can register, regardless of points. This year it was $115 I believe.
There's lots of 'truly bad' players in the Am worlds. It's worth the experience anyway. Just being around worlds this year and not playing was so much fun, can't imagine how awesome playing will be. Don't worry, I'll be on the bottom card with you