No, pre-Lion Macs unfortunately can't have access to Photo Stream.
Evidence: it is an iCloud feature, and iCloud setup specifies:
Make sure your Mac is running the latest version of OS X Lion.
If you don’t have Lion installed, you can purchase it from the Mac App Store.
It might, IMHO, appear under one of these two scenarios:
Apple brings iCloud to Snow Leopard in an OS update.
Unlikely, it is indeed a commercial argument for updating, and if they had thought it was strategic for them to increase the userbase, they could have done it earlier already.
PhotoStream is made accessible through the excellent iCloud webapp.
More likely, but still not so much in absolute terms. They know we'd have syncing tools for all OSs the day after it is released, and they'd lose the commercial advantage given above. Maybe when they know almost all potential upgraders have already upgraded…