I find it strange that he doesn't explain his confidence with at least a general description of the way in which he thinks the proof will fail.
I suppose he could have safely claimed this with Wiles's original proof as well, since it did have at least one non-trivial flaw that required amending. It's quite possible that the proof contains flaws, but will still hold up in the end, because the flaws can be corrected. But I feel that's a bit of a lame gamble.
Well, but Scott Aaronson wrote: "If Vinay Deolalikar is awarded the $1,000,000 Clay Millennium Prize for his proof of P≠NP, then I, Scott Aaronson, will personally supplement his prize by the amount of $200,000." So I suppose that any non-trivial flaw which will be eventually fixed by author of the paper himself is not a problem here.
Of course, but that's an even larger gamble, if you don't have at least a very specific hunch about the way in which a proof will fail. All in all, this announcement by Aaronson seems rather rash. I don't understand why he would do such a thing.