Exchangepedia reports that iOS 4 also puts a strain on Exchange Server, which could cause performance issues for Exchange 2010/2007 Client Access servers as well as Gmail, which use Exchange ActiveSync for email access.
The fix offered by Apple increases the default timeout parameter to 240 seconds, which should allow easier syncs for users. According to Apple's knowledge base article on the subject, Exchange Server administrators should also see an increase in performance.
Once the configuration profile downloads, tap Install, enter your passcode if necessary, and tap Install Now after the warning about the profile being unsigned. Tap Done and reset your device. Exchange should sync properly going forward.
You can find the profile after you install it on your device by navigating to Settings > General > Profile.