Slate has a really interesting take on why Olympia Snowe should follow Arlen Specter's lead and jump to the Democratic party. Using a social-Web-style cluster map where Senators are connected if they vote together 65% of the time, it's clear she's pretty much a Democrat already:
Slate didn't add an embed code to this animated map (which is really annoying because they're usually more Web-friendly, being a Web-only magazine and all) so I took a screen capture while mousing over Snowe's dot. As you can see, she's tied to far more Democrats than Republicans already. She's the mirror image of Ben Nelson, the center-right Democrat from Nebraska.
Hell, by this measure, Susan Collins (the the left and slightly below Snowe's dot) is the only true independent left in the Senate. Specter was far more right-leaning than her, but made the jump because of political circumstance. But if you think back to the amount of shit Specter, Snowe and Collins took for supporting the stimulus bill, you can see why they are gravitating away from the Republican Party. This was all only a matter of time.
TPM's "Day in 100 Seconds" video gives you a good look at the reaction from both sides. Please note the Fiddling While Rome is Burning attitude of Michael Steele, who is RNC chair, and thus, in charge of making sure his party grows its electoral clout, not piss it away: