That was quick! Looks like there's already some data on the question of what impact Bloomberg would have.
Bottom line: Bloomberg's impact depends entirely on who the Dem and GOP nominees are, but it's clear that in more cases, his entry actually flips the states from red to blue than the other way around.
One other interesting point: The polls suggest that the two candidates who would be most hurt in a general election by a Bloomberg entry are Mitt Romney and, surprisingly, Barack Obama. And Bloomberg flips states when either of those two are nominated — at least for now.
I'm not that surprised by the Obama hit. Obama may actually be the most likely of the three top Dems to have some crossover appeal with Republicans who are sick of where their party has gone in recent years. Hillary is poison to the GOP while Edwards is running the most openly progressive campaign. But Obama speaks a language of reconciliation that may appeal to dispirited Republicans.
Bloomberg may give them a more acceptable alternative in the "I want to send a message to my party" dynamic.