Can't recall any other drops, but his general keeping's been shoddy for the most.
Going through the cricinfo commentary for the tests agains the West Indies when WI were batting searching for "Prior" found the following chances put down or drops:
None in the 1st test. His keeping was generally praised especially to an atrocious Harmison. Made one mistake standing up to Collingwood that cost four byes. No drops.
Second test, second innings:
2.4 Sidebottom to Gayle, 2 runs,
dropped. Prior dived across first slip for that one, his first mistake for England, snatching with his left glove after Gayle was squared up there
Also let 4 byes past on one occasion that was tough but stoppable. Again generally praised for his keeping to Harmison and Plunkett.
Third test first innings again generally praised for keeping to wayward bowling but not quite as much as in the first two tests. One occasion of 4 byes deemed stoppable and one extreemely tough chance down legside that he couldn't reach.
Second innings:
68.6 Panesar to Chanderpaul, no run,
missed stumping from Prior. Completely beaten Chanderpaul - in fact, it turned the other way! Monty has a doosra, and Prior didn't pick it. Collingwood was quick to save any runs by diving behind Prior
82.3 Sidebottom to Ramdin, FOUR,
dropped. A hard chance for Prior, diving away to his left as that's strangled - he goes with two hands and the ball just clatters of the glovetips
Other than that didn;t let any easy byes past him and saved several athletically.
Fourth test, first innings again one incident of 4 byes that was attributed to be his fault (got his gloves to it but only parried it to 3rd man) but other than that reckoned to have kept excellently, including up to the stumps on occasion when Hoggard and Sidebottom were bowling.
Second innings once more a case where 4 byes were his fault, this time a shocler apparetly. Other than that kept well, not as praised for saving byes down legside as much but probably because Harmison wasn't giving him as many opportunities.
So there you go, in 4 and a bit tests, Prior has made three drops and one missed stumping, plus another chance that would have required a tremendous effort but could in theory have been caught nonetheless. Now, whether that is par-for-the-course for a decent international wicketkeeper I have no idea, but those are the figures.
Of course this is all dependant on agreeing with the Cricinfo commentary, but seeing as I don't think any of us have the time nor capacity to go through the entire tape of each innings ourselves, it's the best we have.