The only 2 I saw listed were the Gold Coast and the Orleans, so I tried them last week. The Gold Coast birthday promo is no more. At the Orleans, you have your choice of 5 balloons to pick and I got one for $10 off the buffet...unfortunately we had just eaten there with points so it was useless. Other options were mostly free play or points ($10 or 1,000 points).
Obviously, the best birthday offers come via the mail which I took advantage of at the Paris and Sam's Town.