It doesn't matter. It's personal perference.

I'll agree with Chris. While it probably doesn't matter in casual print whether you capitalize "Happy Birthday" or not, it's certainly not necessary. Technically speaking however, I believe it's incorrect to capitalize the phrase.

That's just input from a native English speaker without much in the way of formal training.

I capitalize it, whether technically correct, or not :-)

If you take a look, here, most of the birthday cards are also capitalize:

http://images.google.de/images?hl=de&ie=ISO-8...

Just to add my two cents:

If it's just saying, "Happy Birthday," then I'd capitalize; but if it's in the middle of the sentence, i.e., "I hope you have a happy birthday," or, "I want to wish you happy birthday," then I'd keep it lowercase. As everyone else said, though, it doesn't really matter.

Two more cents:

I agree with Charlie except that there is a grammatical difference between "Happy Birthday" and "have a happy birthday."
The former is like a greeting or even a command. I'd treat it like "Merry Christmas" - same structure. "I hope you have a merry Christmas" fits too.

Agree with Charlie and Bill -- you can capitalize it if it stands alone, like a heading or title (as on most greeting cards), but you shouldn't in the middle of a sentence.

I have to revise my own post, because I wouldn't capitalize it in a sentence, either - however, I invariably start my birthday cards off with: Happy Birthday, name !

I have to revise my own post, because I wouldn't capitalize it in a sentence, either - however, I invariably start my birthday cards off with: Happy Birthday, name !

I always capitalize Name, too.

SCNR :-)

@Tom-AE - LOL! Me, too :-) REALLY! Honestly! :-)

If you are writing a card, you should capitalize both words because they are formal greeting. However, I suppose it comes down to a matter of preference in the end.