Torontoist, so far as I know, beat the traditional media to this (very plausible) speculation.
For the record, Giambrone is actually my representative in City Council, as the incumbent for Ward 18 in the south of the Davenport electoral district as well as the chair of the Toronto Transit Commission.
Since the day that Mayor David Miller announced that he would not be seeking reelection, Adam Giambrone has been included on any list of potential candidates to replace him: to replace Miller as mayor, and to replace him also as the standard-bearer of the left at City Hall. We have just learned that Giambrone will soon make good on the rumours and the speculation and launch his candidacy in just over a week, with a party at Revival. (It is expected that he will be filing his nomination papers at City Hall earlier that day.)
While many have greeted the prospect of Giambrone's campaign with some scepticism, citing his youth and growing frustration among many Torontonians with the TTC (of which he is chair), it is also the case that Giambrone enters the race with several organizational advantages that make him impossible to dismiss. He has a long history of involvement with organized labour (he was, among other things, president of the federal NDP from 2001–2006), and thus will be well-placed to make a bid for the majority of union support. It is also rumoured that Giambrone has secured the services of John Laschinger, the formidable political organizer who helped shepherd David Miller to victory in 2003 and 2006.
An Angus-Reid–Toronto Star poll released ten days ago had Giambrone in second place among decided voters, with 17% of the vote compared to 44% for current front-runner George Smitherman. It's a long time until election day, though—nine months can be several lifetimes in politics—and those numbers will undoubtedly change many times between now and then.
For the record, Giambrone is actually my representative in City Council, as the incumbent for Ward 18 in the south of the Davenport electoral district as well as the chair of the Toronto Transit Commission.