I reached a similar conclusion a while back and actually ended up writing an app that would automatically wish friends happy birthday when I was trying to get to grips with the facebook api: http://blizzardsystems.co.uk/dr/node/44
As the post mentions, the worst is when people can't even take the time to write an original birthday wall post and just copy/paste one from somebody else. I've had strings of the same message over and over again on my wall. I've since made my birthday private.
I have a friend who once spent an entire month logging in every morning and setting his birthday to the current day. And yes, he did get many, many erroneous birthday wishes from people who didn't notice anything was up.
As for me, I don't list my birthday to anyone. I don't like receiving those post floods. (In fact, really, the only information I list is contact info, since I only use FB for keeping my contacts up to date.)