I was talking to a knowing Democrat about John Sabini’s recent run in with the law, who pointed out that although it’s never good to get busted for DUI, there's no obvious existential threat to Sabini's electoral career. Sabini had a couple of close elections, in 2006 and 2004, but those were primaries – challenges from other Democrats, not Republicans.
So before we can tell how serious this is, politically, we'll have to wait and see which Democrats are prepared to try to take advantage of the situation in an election year that will determine control of the state Senate.
UPDATE: I asked Sabini's spokesman Shams Tarek about a rumor that his boss may want to run for his old City Council seat, which is already drawing interest a number of other likely Democratic candidates.
Tarek dismissed the idea, saying that Sabini is "staying in the State Senate and helping his fellow Democrats take the Majority."