From the New Republic via the Political Wire:

"This isn't the first time that Bloomberg has privately flirted with a 2008 bid. But what makes a Bloomberg candidacy look increasingly real is that he has also begun to think about the mechanics of running. New York p.r. eminence Howard Rubenstein recalls Bloomberg putting a price tag on his Oval Office ambition at a dinner party in April: 'I could easily put up half a billion,' the mayor had said, naming a figure over one-third higher than the Bush campaign's spending in 2004."

"Bloomberg has suggested that, if he runs, it would be on a new party line of his own creation. No cold days in Iowa, no small rooms in New Hampshire. He could afford, like Ross Perot, to set up a petition drive to secure ballot status in late 2007 after the Republican and Democratic candidates were clear... Playing to his strengths as a technocrat, he would run on competence and nonpartisan management -- the style, over the substance, of his politics."

I would add that Bloomberg is viable (or "dangerous," depending on how you look at it) also because he's very well-versed in information technology. His fortune was built on providing services we now take for granted as part of the internet to the financial sector as much as 20 years ago. His chief accomplishment as Mayor was implementing 311, a catch-all service number for the city. It's been a rousing success in an otherwise bland/middling (or, to spin it brightly, "competant") term in office.

Assuming he'd take that kind of savvy into his campaign, he'd probably have a fairly sophisticated communications strategy; at least as innovative as Perot's infomercial timeblock buys were in '92. This plus a huge potential war chest would make him viable enough to steer the national debate.

My guess is he probably won't, but somebody might. Assuming neither party produces a nominee who breaks with the status quo, the opportunity will be there to make a splash. Lots of people pretty generally upset with things, looking for alternatives...