There are a lot of good experiences like the one above, but please don't *expect* it or you may be disappointed.
Case in point - last Sep we went for a week with a party of 10. I was at POR, the rest at GF. My bday as well as my little 4 yo friend's bday.
While making the res, the CM asked why I was going, told him I always go to WDW for my bday. He made note of the date (he asked me for it, and made a pretty big deal about it on the phone). A few days before my actual birthday, my friend gave me a gift, in a gift bag which said 'happy birthday'. I removed the gift, but left the bag in my room. On return to my room I found a card signed by lots of characters, wishing me happy birthday. I think that it was initiated by the room attendant, who would have seen the bag, since it was not yet my birthday (that fell on a hurricane, but that's another story!)
We went to O'Hana for my birthday dinner (day after hurricane) and my friends had ordered me a cake. It was delivered with nothing else (ie no singing, etc, not that I expected/wanted anything else)
The next day was 4 yo birthday. We went to CG (ok my friend booked this backwards, but so what!) Here we ordered her a birthday cupcake, and the servers were very nice, but again nothing 'special'. Nor did she get anything in her room, etc.
I'm in no way disappointed - I just want to point out that it's very hit and miss, so don't expect something. Be surprised if it does happen. I suggest planning something special yourself to ensure no disappointment.
As others have said in other threads, a LOT of people have special occassions at Disney - weddings, honeymoons, anniversaries, birthdays, etc. The magic for me comes in the element of surprise....