Last summer was the first time he'd had an extended spell without a shocking game. I myself have said many times that I am not prepared to believe a player has improved on the basis of a single season's work, they must demonstrate good-quality performance for longer than that, more so than ever having been truly awful beforehand.
And yes, Prior 2007-2008/09 was indeed truly awful. It is quite true that in 10 of his 13 Tests (yes I know officially it's 14 but given that 1 of these lasted 14 deliveries, none in which he participated, I'm sure you'll agree it's irrelevant) he'd done not-that-much wrong, but in the other 3 he was as I say so dreadful as to beggar belief - honestly as bad as I've ever seen any wicketkeeper be in any Test. It is not at all acceptable to be that bad that reguarly as a Test wicketkeeper - games that bad are the sort that a competent wicketkeeper might expect to have once in 40 or so, and an excellent one basically never.
Prior will have to keep-up his form from the 2009 summer for a little longer yet for me to be convinced he's a long-term option. If he is, great, because I've few doubts remaining over his batting - he's excellent against moderate seam, excellent against spin and seems competent enough against good-quality seam. But I think anyone claiming he was anything other than very poor indeed as a wicketkeeper in his Test career prior to the 2009 summer is kidding themselves.