I think this is brilliant. Shouting "Happy Birthday" is really the most consistently used feature of Facebook, it makes a lot of sense to turn it into an opportunity to sell something - and lower the barrier for people to send gifts (there's a lot of steps involved in getting a greeting card and addressing it and buying a stamp...)

And they could size this up over time - like, ten of your friends from college are going in $1-each on sending you flowers after your recent breakup. I think people would actually do that.