Getting married online isn't a new but proposing on the microblogging service called Twitter is a novelty. This Wired article documents two such incidents. The most recent was Max Kiesler's proposal to Emily Chang.
Max Kiesler's sweet tweet at 3:13 a.m. Thursday: "To @emilychang - After fifteen years of blissful happiness I would like to ask for your hand in marriage?"
Emily Chang's reply, a minute later: "@maxkiesler - yes, i do."
The earliest known documented marriage proposal on Twitter happened on March 2nd, 2008. You can see the proposal tweet here.
The proposal: "@stefsull - ok. for the rest of the twitter-universe (and this is a first, folks) - WILL YOU MARRY ME?"
Sullivan's reply: "@garazi - OMG - Ummmmm... I guess in front of the whole twitter-verse I'll say -- I'd be happy to spend the rest of my geek life with you."
Just two proposal tweets are known so far but with Twitter's popularity growing there are bound to more.