This is the 3rd pregnancy that I've dealt with this, but the first time was the worst since I thought nothing could be done, and so tried nothing. With the next pregnancy, my midwife said that Tylenol and heat were the only things that might help. However, I also had an anterior placenta, and in my attempts to avoid having a posterior baby, I began doing the things recommended by Spinningbabies.com. (Simple things like sitting with both feet on the floor and keeping my knees lower than my hips -- even when that meant sitting on a stack of books in the car.) Within a week or so, my SPD pain was lessened a great deal, to the point that I had more pain free days than not. Not to mention, I avoided a posterior baby! With this pregnancy, the only times it bothers me are when I'm being lazy about my posture and movements, but shortly after correcting myself, it improves.
As far as affecting delivery, it shouldn't. However, discuss it with your doc or midwife ahead of time. You'll want to be a bit more careful with choosing your pushing position -- nothing that has your legs widely spread is going to feel great. I would definitely not recommend the semi-reclined position for pushing, especially not with your legs spread in stirrups. My last baby was expected to be large, so even before he crowned, they began treating me as though he were stuck. They shoved my knees to my ears and a nurse jammed her hands into my pelvis (apparently to keep him from getting stuck on the pelvic bone). I would not recommend this to anyone with SPD unless it's a true emergency and there is no other option. Because of my delivery, my SPD was worst postpartum and lasted until my son was just over a year old. If only I'd insisted on staying on my hands and knees . . .
Well, I just found this link which says most of what I was trying to say, only more concisely.
http://en.allexperts.com/q/Childbirt...ion-breech.htm