NOTICE: iTunes app update probs may be caused by dups
 

If you get the 'application not installed because unknown error 0xE800002E' problem, it may be due to multiple copies of an app being on your iPhone.

Try deleting problem apps, then shut down and restart your iPhone. Look for those SAME apps and delete again. Shut down, restart. For me this cleared the problem of installing apps from iTunes.

[url]http://discussions.apple.com/thread.jspa?messageID=7835864[/url]