Missing Exchange calendar in iCal
I had exactly the same issue and thought it might be the Accounts sqlite database file that was corrupted. So i removed the exchange account and then:

in: 
~/Library/Accounts there is:

Accounts4.sqlite
Accounts4.sqlite-wal (write ahead log file)
Accounts4.sqlite-shm (temporary file)


i did the following as another admin user on the box (to be sure no files were open). This is all on your own risk by the way!


first a backup

cd useraccount/Library/Accounts
mkdir backup
mv Accounts4.* backup/

Rebuild Accounts file:

sqlite backup/Accounts4.sqlite ".dump" | sqlite Accounts4.sqlite



Then log out and log in again on the right account
Add the exchange account again
open ical and wait some time to give ical time to sync. My account took approx. 10 minutes!