Man, there are some GREAT posts on this thread. Very insightful!!
I agree with Jess that most women don't propose because men, on average, seem to be a little slow on the marriage draw compared to their partners. Men need a longer period of time to give up their cave and "freedom," as they say (hell, a lot of women need that time, too... I do!) Rodney and Poppinjay had really nice explanations of the man's side.
I see no moral or social reason against a woman proposing, but proposing when the man isn't ready is asking for the shit to hit the fan. I think it has to do with the fact that men might see marriage as losing control (or a degree of autonomy/non-responsibility, as the guys said above)... so it's really important for them to feel okay with that decision of losing autonomy, which means it comes down to their decision and their timing, essentially.
Onodrim: I agree with your reflection on the ring stuff. I've always liked the idea of both people having engagement rings (yay, SnowyOwl!)... I mean, both people end up with wedding rings, so why not both have engagement rings?... (or both people have nothing, I guess, but I do like the idea of a ring as a symbol.) Maybe it doesn't have to be an engagement ring... it could be some other piece of jewelry, or just something else physical/symbolic that both people agree on (beer and TV

). Hell, it could just be the act of moving in together... for me, that would be really important. I don't like the idea of living together as being merely a "trial period." I like the idea of it being a sign of commitment, a final step before marriage. Anyway.