Pursuant to a request for the same thing in the Elements forum, I Imported a PE4 Project into PP2. Everything came in, except one Title in my test Project. Other Titles did fine. This one was a test of Type on a Path, and it showed as Media Offline (you know that multi-language red screen, that we all love so much.) In my test, I had mux'ed AV material (all DV-AVI), stills, Titles and SmartSound Track. Everything, except as noted above, Imported perfectly and played perfectly.
One note: when going to Import, you will need to twirl down to "All Files," for PP to see the PREL files, otherwise, they do not appear.
This one surprised me. Now, I do not know about other flavors of PREL (say PE3), and also do not know how they might Import into CS3, or CS4.