Recently John Romita, Jr announced on Fanboy Radio that he would be taking a sabbatical from Amazing Spider-Man art duties to do other projects. This was followed up by an official announcement from Marvel and by John himself at the Spider-Man Message Board. You can read more about it at Newsa rama.
In a follow-up on the Spider-Man Message Board John dropped in with the following update in regards as to whether issue #508 would be his last for ASM:
I thought it was #508...I'm wrong and I don't know which will be the last....spread that news, please!