I noticed that Adobe now has downloadable trial versions of all of the CS3 versions of their software now, up on their website. Be prepared to wait a while though as these file sizes are somewhat daunting. In my opinion, it's worth the wait though :)
If you want a trial of the entire Adobe Creative Suite, it appears that Adobe is "selling" a trial version for 10 bucks. It actually covers the cost of the DVD that they will send you -- trust me, you don't want to be downloading the installer for the entire Creative Suite.
Adobe's trial software are full versions that are valid for 30 days of use from when you first install them. This gives you a way to fully evaluate the software on your system before you decide to upgrade or purchase.