(I assume, that if the latest QuickOffice doesn't handle .pps files, then QuickOffice would have to add support for .pps files in some future version, and that there's nothing you can do as a user except to wait, or use a PC/Mac for those files, if you can't get access to the original .ppt/.pptx otherwise.)
I think you can use another symbian application to open any office files, but I think that QuickOffice can read and edit any files made with MS Office on your computer, but not "play" presentation.
You can use any other reader supported presentations and I have there one link I've found. http://code.google.com/p/officereader/
But this app support only files created with OpenOffice.org and I don't know if you can run presentation.
I hope this would be helpful.