It seems like a long way away, but I just started thinking about this, because I know my mom is going to be asking soon!
When I had my DD, my mom was invited to the birth, but sort of because she REALLY wanted to be there. I wasn't really sure how I was going to feel and such, and was more inclined to just have DH and I, but I did invite her in the end. However, DD came very fast, and she wasn't able to make it in time (fast birth + long drive for her), and arrived about an hour after DD was born.
For DS, I invited my mom again, and was more ok with having her there. This time, we were very alert to labor signs, and she had plenty of time to get there. It was nice having her there in some respects, but in others she was very distracting. She was trying to call in to work (she was in a job where calling out for the day was extremely discouraged, and could potentially cost her the job) so I was literally in transition, pacing around the room, and she was in the corner of my room making calls. Earlier in labor, when I was just kind of resting/breathing thru contractions, any time I opened my eyes, she was right there, studying me, which was not relaxing at all! It was special to have her there and see her hold DS for the first time when we was so tiny, though.
So this time around, I am really thinking hard about what I want. On the one hand, she had both me and my sibling naturally, worked in a labor support capacity at one point in her career, and is a highly skilled medical professional, and if anything went wrong (we are birthing at the hospital), I would totally trust her to interpret what is going on, and give me advice that would support what she knows my birth preferences are. Plus, she'd never allow any kind of medical bullying - she'd rip someone's head off before she'd let them give me a hard time :)
On the other hand, when I am laboring, I have learned that the only person I really want around me, touching me, or helping me is DH. It would be nice if it could just be us laboring together, and not having to share any of that first bit of special family time with anyone else.
Further complicating all this is that my solo-practice OB is taking her only vacation of the year on, or a little after my due date. I love her, and she is extremely supportive of natural birth, BUT if I go past my date, and get stuck with OB-du-jour, I think I would want my mother there, because of her aforementioned skills. However, it seems sort of rude to say "I don't want you at the birth, unless of course, I want you there for medical support"
Anyway, those are the thoughts swirling about in my mind these days! Who are you inviting to your birth?