Check your account settings and make sure the two boxes on the left are checked to receive notifications from PSN. I got my PSP Go on Christmas as well, and after calling three times over the past few months I finally got 'em last week. I wish I could remember the lady's name who finally got the ball rollin' for me 'cause I'd give her a big SALLLLUTE and many thanks! :-) I was told previously that they send them out on Wednesdays too, but I got mine on a Monday.
You should get an email before getting the voucher that asks for info such as: serial #, proof of purchase, and such. When you send that in, the voucher isn't far behind.