Mac OS Lion UnityPro 3.4.2f2 preferences cannot select asset server diff

MacOSX Lion

Asset server client installed and functional, connected to remote asset server. Have checked-in revision of a project. IOS builds, have Xcode 4.2.1 installed and functional. Unity preferences/general allows selection of external script editor, image application but asset server diff tool pop-up is inoperative, consequently "No supported asset server diff tools were found"

Nowhere in Unity documentation does it describe what is mean by "installed"... XCode filediff app is in place and functional, TkDiff in in applications folder and functional.

confirmed that my Xcode installation on Lion has same components in same places (co worker also with Lion/XCode 4.2 which works as it should) - searched through logs looking for evidence of some error thrown by Unity, no luck. Apps in same place on functional/non-functional installations., co-worker who has the same setup and the Xcode FileMerge app (which is in the same location as the one I have) shows up in his Unity preferences. The "usual" location for this is /Root/Developer/Applications/Utilities/FIleMerge.app - our installs are alike. I've looked the logs (via console) for footprints of some Unity/system/UI error - nothing fishy there.

How to convince Unity preferences that file selection popup for asset server diff tool should be enabled?

I deleted Unity folder (emptied trash) and did re-install (UnityPro and IOSPro serial numbers) No help.

I created a new empty HD partition, installed virgin copy of Snow Leopard and booted, picked-up all Snow Leopard updates, installed Lion onto that HD partition, picked up all updates, contacted Unity and had my license on original HD/OS de-authorized, downloaded Unity latest from their site and installed. Same problem, user client diff tool does not allow selection. Downloaded 3.5 Unity Beta - same problem - since I have pristine/virgin installation of OS I can only conclude that something is missing from the Unity installation, since I have introduced no conflicting pieces on the OS side.

To date (after submitting bug via Bug Reporter) have received zero feedback from Unity support.

UPDATE UPDATE UPDATE UPDATE

The most recent OS update (10.7.3 Feb 2012) to Lion does indeed now offer me

Apple File Merge

as an option in the Unity Preferences pane, Asset Server diff tool selection

(BUT)

(1) that is the only option offered, when I have TkDiff application as well which is NOT shown (2) selecting the "Compare" button in the Asset Server window now displays "Initiating Compare..." dialogue briefly, then nothing, dialogue is closed - nothing happened, filemerge is not launched, no comparison windows open

I tried this with and without Apple filemerge application already running.