I've had this same behavior from App World on several occasions. I begin to download an update, which ends up being the exact same version I'm currently running, and the download fails after getting about 1/5 into it. I know it's the same app version too, because after eventually getting it updated through App World I'm prompted to replace version 4.2.11 with version 4.2.11, or whatever. And after rebooting, App World still shows an update available for that same app! That's usually the sign for me to delete it and move on.
Finding an external link is the best way to get an app that fails to download through App World. Quitting and restarting App World, then restarting the download process gets me farther along each time before failure, but it gets the download done after 3 or 4 cycles. I still call that a FAIL.