[whpalmer4]

I suppose it would be even more helpful to tell you how and where to make that one-line change, right? :-)

In the Finder, select the OmniFocus application. Right-click or use the gear menu and select Show Package Contents. Double-click on the Contents folder in the window that opens, then double-click on the Resources folder in the next window that opens. There's a file called MailActions.applescript in that folder. Make a copy of it, suitably named and located (you'll need to point your new Mail rule at it, and if it isn't buried in the application, you may not have to recreate it each time you update the application(*)). Use the Applescript Editor (aka Script Editor if you are using Leopard, IIRC) to change the script:

Before:

		set theSubject to subject of theMessage
		set singleTask to false
		if (theSubject starts with "Fwd: ") then
			-- Whole forwarded messages shouldn't split.

After:

		set theSubject to "From:" & theSender & " " & subject of theMessage
		set singleTask to false
		if (theSubject starts with "Fwd: ") then
			-- Whole forwarded messages shouldn't split.

Click the Compile button to make sure you haven't made any typos, then save the file. Make a new Mail rule that runs this script instead of the normal one when it detects a message that should have this handling. Make sure you've got the sending addresses listed in the Mail preferences in OmniFocus.

(*) It would still be a good idea whenever you download a new release of OmniFocus to compare the MailAction.applescript file with your edited one, in case they have made any changes.