If you recently upgraded to iOS5 on your iPhone and are having issues with adding a new Exchange account where after adding the account OK, you encounter a wrong password prompt continuously, try resetting your network settings.  Go into settings, general, then reset, reset network settings.

You will have to re-input whatever WIFI settings you had.  However, this fixed the issue on two iPhones that I own, one is an AT&T and the other a Verizon.