Yup. Same problem for me as well.
My original exchange account stopped syncing email, so, I tried to reset it by unchecking the email option and then re-checking it. I ended up somehow with 2 corporate exchange accounts. One was the original with the usual server info, etc. the second comes up with the grayed out username and asking for a password that I can't seem to get rid of. I've been messing around with it for a day.
Searched bunches of other forms to no avail. this is the only one I've found where the problem seems to be clearly identified at least.
I managed to fix the issue on mine today. Same symptoms, greyed out username field.
I went to the Contacts app and hit menu button | more | manage accounts. Then changed properties on my "fake" corporate account. Then scrolled to the bottom and hit "remove".
Hope it works for you guys!
I tried EVERY permutation probably 100 times from every angle.
In the end, I gave up and did a factory reset. Restoring is pain, if only because of setting up my default ICON layouts and browser defaults. I wish apps would allow me to remember any setting!