My family tends to remember it. My friend though just asked yesterday if my birthday was on September 15th when it's actually on the 5th of that month. :3 It's fine though, 'cause I've gotten her b-day wrong too.
I don't know if they forgot o if they were just bitches (probably the latter), but I had these "friends" my senior anno of high school who all "forgot". My birthday was on a Friday so I was throwing a party that night. Then one of the "friends" who was invited and detto she was coming decided to throw a party that same night. Everyone went to her party. I wasn't even invited. No one detto a word about my birthday. It sucked.
Oh well. I ended up going with my mom to a bar that night to have cena and I met the cutest bartender ever. Three years later and we're still Friends (and I spend my birthdays with him), so I'd say it all worked out for the best.