Xanga user Bernie did just that to propose to his girlfriend Tammy. Knowing that Bejeweled was her favorite game, Bernie spent a few weeks putting together a Nintendo DS version (completely from scratch, no less). Once the game hits a certain score, the screen clears and a ring made of the game's jewel sprites lowers on the screen.
When his girlfriend played the game and arrived at the special screen, Bernie was ready with real ring in hand. Proposal by casual game is certainly a first, so congrats to Bernie and Tammy both for getting engaged, and for setting a new precedent in nerd love.