WaPo:

...McCain applied to be certified for federal matching money last year, when his campaign was running on fumes. But unlike former North Carolina senator John Edwards, McCain never actually took the federal funds. He was merely preserving that option. Once his campaign started to take off, he wrote to the FEC requesting to withdraw from the program.

The reply from the FEC, [short version: no] dated Feb. 19 and released by the FEC today, will almost certainly come as a shock to the campaign, which assumed McCain had withdrawn from the public system two weeks ago and was now free to spend money without regard to federal limits.

The reason this matters so much? Life within the public financing system, in which candidate contributions are matched with federal funds, comes with severe spending restrictions.

Restrictions that neither Obama nor Clinton have, because they did not take matching funds with their accompanying spending limits during the primary season.

NPR:

Then, McCain's campaign fell on hard times. He applied for the primary-season matching funds. The Federal Election Commission approved the application, but McCain never cashed in his certificates.

Instead, McCain went to a bank and received a line of credit. As Potter notes, McCain did not put up the public-financing certificates as collateral.

But then the McCain campaign renegotiated for a bigger line of credit. The loan documents said that if McCain lost the New Hampshire primary and finished more than 10 points behind the winner, then he would have to stay in the race and reapply for public funds, which would become the new collateral.

Lawyer Cleta Mitchell has long criticized McCain's role in overhauling campaign-finance laws. Pointing to the terms of the loan, she accuses McCain of gaming the system.

"They got another million dollars with, really, no additional collateral, other than a promise that if he didn't do well, they'd go get it from the federal government," Mitchell says.

Insert Campaign Finance Reform joke here, if there are any. McCain says it's totally legal, but that's like my neighbor saying his dog's poop doesn't stink and actually makes my lawn look better. He flip-flopped on whether he was going to use the funds, and then used the promise of matching funds as collateral for loans. It simply stinks, Senator.