I did upgrade to 10.6.1 so don’t know if that makes a difference.
set zzz to AppleScript's text item delimiters
set AppleScript's text item delimiters to paramHere
set theItems to text items of textHere
set AppleScript's text item delimiters to zzz
Yes, I have noticed in the meantime that the statement does indeed execute correctly. I was misled by the fact that on my machine in AppleScript Editor the word “AppleScript” and “text item delimiters” did not adopt the correct context colouring (for syntax which is recognized) once compiled. This is also visible in the screenshot that you posted (the words remain purple, i.e. are not marked by the Editor as recognized).
Since the execution is correct nevertheless, this is a lesser problem.