The first person I told was my now-ex girlfriend (long story, not related to the diapers). I left a note on her keyboard as she normally checked her email in the morning. Of course that was the one day that she didn't. I almost called it off at that point. I tried again the next day, leaving it somewhere she'd be sure to find it. It worked out well, we were together for another 3 1/2 years.
My current girlfriend I told directly. It wasn't easy, as I'd only been dating her for a few weeks at that point. We'd been talking over MSN for a few months before we got together, face to face, though. I was very worried that she wouldn't be accepting of it, but she's very supportive. She'll wear diapers voluntarily and has even changed me a few times.
Neither of them has ever said anything to their friends. Tell your partner about your desires is definitely something you need to have trust in your partner for, but I've had very good experiences so far. My first girlfriend was worried that I was breaking up with her after reading the first half of the note, and then got to the second part and was very relieved that it was something she could deal with and help me through. My current girlfriend, as I said, has been very supportive as well.
I met both of these people in the old fashioned way, though, I'm really terrible at meeting people so, in both cases, they kinda found me. My first girlfriend was the high-school sweetheart, and, strangely, she set me up with my current girlfriend a few months after we broke up. It helped that my current girlfriend has a few kinks of her own so she's definitely not a prude.