My DD was 2 1/2 when we took her. When I made the appointment, they didn't ask her age, so I didn't say. When we got there, I told our FGMIT her age, and she said that was fine, and even that she preferred the very little ones! (she was probably just being nice).
My take is that the 3 year "rule" is more of a guideline. I KNEW that my daughter would be in heaven at the BBB-- she loves dressing up and having her makeup and nails done, and she is used to sitting for me while I comb/style her hair daily. She is also obsessed with Cinderella, and mature enough to understand that she was going to "look like Cinderella" afterwards. She did amazingly, had a great time, and is still telling people about it 2 months later.
That said, I know other 2 1/2 year olds who would rather eat spinach than let anyone (particularly a stranger!) comb their hair for 15 minutes. Also, some 2 year olds may not understand the whole concept, and therefore, be able to actually enjoy it.
You know your daughter best. If you think she would enjoy it, I say go for it.
Here are some pictures from my DD's session at the MK: