It took a while to figure out, and like others, I went into Keychain first thinking that was the source of my problem. I tried to make sure OmniFocus had access to my new iDisk keychain password, and manually added it, but to no avail. I finally fixed this problem doing the following:
(1) Open Keychain from the following location: /Applications/Utilities/Keychain
(2) Locate the key with the following properties: Name - idisk.me.com (<username>@me.com) ; Kind - Internet Password ; Keychain - login.
(3) To be certain you're working with the correct Key, double-click on the key to bring up it's attributes.
(4) Select the Access Control tab
(5) If the Access Control list ONLY includes OmniFocus, you're definitely working with the correct key.
(6) Close the key, then right click or ctrl+click on the item.
(7) Delete the key from Keychain.
(8) Launch OmniFocus and go to the menu OmniFocus>Preferences>Sync
(9) Click the "Show Clients" button. When doing so, it should prompt for the MobileMe username and password. If it does not, you've likely deleted the wrong key.
(10) After entering the correct username & password, press "Sync" and it should connect and sync to the proper MobileMe iDisk location.
Hope that helps! :)