My girlfriend learned when I happened to mention it one day, having thought I had already told her. While this was news to her, she wasn't taken back or even surprised. By that time we were already serious, and perhaps she simply didn't know anything about Asperger's. Anyway, I explained briefly what it was and how it affected me, citing things she would have already noticed about me.
I'm very fortunate because she told me that she wouldn't want to change me for anything. To the right person, AS is simply a part of the one who they already love, no more or less significant than any other.