To me, this really depends. My best friend of 16 years and I rarely remember to catch each other on each other's birthdays. It's usually a comical, remorseful call a few days late, and a belated gift, but no less thought out.
However, if I had a good friend whose birthday I always remembered, always was there for and always made sure to acknowledge, and she or he forgot my birthday completely, I would feel kind of let down. This is of course, unless there's a good reason. Sometimes life just gets in the way of everything.