I talk about my birthday a lot leading up to it, so yeah, it'd be hurtful if an SO forgot. I had a first date on my birthday once and that guy even gave me a gift -- a second-hand book (went dutch on lunch, though).
My most recent ex and I had the same birthday, and he hated celebrating his while I loved celebrating mine, so we compromised and just celebrated with each other rather than making a big deal of it; my mom visited on another day for a second small celebration that he mostly avoided (we celebrated 2 birthdays together).
With current BF, I made a big deal about his birthday, and if he didn't at least get me a gift or do something extra-nice I'd feel pretty slighted. And I'd mention it. Not in a fit-of-rage way, just "I'm really hurt" etc.