I tried on my laptop where both SQL 2005 and SQL 2008 Standard Edition are installed. And found the following:

--If set the 2005 instnace as primary, it won't let me add the 2008 instance as secondary, as it is beyond its scope.

--If set the 2008 instnace as primary, i will not even be able to initialize the Logshipping, i.e. not be able to restore the fullbackup of the 2008 db onto the targeted (secondary) 2005 instance. Guess, again, the 2005 engine doesn't recognize the 2008 backup.

So I guess the answer would be NO..