I'm not a lawyer, but I think this is a conditional gift. Is it conditional on the acceptance of the proposal or the actual marriage?
Most courts (in other words, most states) construe it as a gift conditional on the marriage taking place. Once the marriage takes place the gift is complete and the ring belongs to her. If the engagement is broken, those courts would then say the gift failed, and ownership of the ring is in the donor (the guy). Obviously, you can override this with a specific agreement, but that sort of takes the romance out of the thing and isn't practical (unless it's a very valuable ring).
This post was edited on 12/6 at 10:25 am