I normally wouldn't post in this forum but, I figured this qualified as an actual announcement. I'm surprised you guys have been so quiet about this. (*edit* ah, now there's something :D)
It seems that anyone who purchases credits with an American Express card (until June 21st) will earn DOUBLE CREDITS on their purchase. I have no idea if there's an "up to" limit to the purchase or not.
IE Spend $10 on your Amex, get $20 in WGT credits. It takes about 24-48 hours to receive your bonus credits.
Don't have an American Express credit card? This works with American Express prepaid gift cards as well!