Welcome Russ!
I am very excited to meet you because I feel like the only SO on the planet that knew about it before we started dating! *doing a happy dance and singing "It's not just me! It's not just me!"*
Let give you a little background info:
I'm Kelly. I have been dating Jocelyn (who is MtF pre-transitioning) for about 6 months. She told me about her condition and about two weeks later I realized that my feelings for this person didn't diminish in the slightest, in fact they grew stronger. I came out of the closet as a bisexual and promptly asked her out. (That's the shortest version I can give you)
To give you my two-cents on this kind of a relationship:
If you both need to be the (girl/boy) remember to treat your partner as such and make sure that it's returned to you (because you both have needs). Jocelyn and I switch doing girl/guy things all the time, and by now it's almost an unconscious effort. Example: I hold her with her head on my shoulder, after awhile, we switch spots. Sometimes I carry stuff for her, sometimes she carries stuff for me. Same thing with opening doors, dancing, paying on dates, etc.
Good communication skills will be your best asset not only in this relationship, but in any relationship. And when it really comes down to it, this is the same as any relationship, it just has issues that most people aren't accustomed to dealing with. Make sure that you make your needs known in a way that is both respectful to your partner and to yourself (don't sell yourself short because you're focusing on your partners' issues). Of course don't ignore your partners' issues either, but I'm betting I didn't have to tell you that

I hope everything continues to go well for you and your partner. If you ever feel like talking to me about anything I'm always happy to help (or try to).