Another theory no one seems to have thought about is...
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. Maybe it's Phoebe who is pregnant. She's just ditzy enough that she might not have made the connection between finding the pregnancy kit and realizing it was the same one she herself used. If it were Rachel, it's hard to understand Rachel playing along so well when Phoebe found the kit.
Maybe Phoebe's brother decided to have another kid and Phoebe agreed to do the surrogate thing again. Of course that would be their fourth kid, so it's not likely, but he and his wife were just weird enough that they might want another. Also, the woman married to Phoebe's brother is now on That 70s Show...
The other reason I have a hard time thinking it's Rachel is that I tend to agree with the theory that she and Ross will eventually get back together, even if it is on the very last show. I can't see a pregnancy working into this. If Ross is the father, then that tends to cheapen the ultimate reunion of the two of them, and it also doesn't make any sense since they have not referred to being together at all and Ross in particular would not be able to hide the fact that they had been together. As for Joey, it is believable that they could do it and even have a relationship, but having a baby together would really put a barrier to any eventual reunion with Ross. For me, a Rachel pregnancy with anyone but Ross as the father is a sign that Ross and Rachel will never get back together...