originally, my bike was #80 because Kellen Winslow Jr. was #80 on the Browns, and had crashed out of his rookie season trying to wheelie in a parking lot. Add the Browns paint scheme, and the joke was anything but subtle to Browns fans.
I went with 80 until I sold my F2, bought my 600RR, got married, and decided to go racing. 524 is my anniversary, and considering that I bought the 600RR (partially) with money we got from wedding gifts, it seemed appropriate. It was also easier to get a 3-digit number with WERA.
I have also run 521 with other racing orgs, because it was easy to throw tape over parts of the 4 and just leave the "stem," to make it a 1.
I talked about changing to 122 (daughter's birthday) when she was born, but she's not old enough to "get" it yet, and I have a bunch of custom "524" decals all over my XR100 race bike, as well as left in my garage