I run both CS3 and CS5 here at my office. I'm going to be upgrading the CS3 to CS5 this month for the $799 price tag now that our dollar is rocking!
Adobe allows TWO activations per license. You can install the software on as many computers as your heart desires, but only two can be activated at one time. You can deactivate and activate all you want.
I used to have only one copy of CS3 running on 2 computers at the office and my home computer. I'd deactivate at work, then go home, activate and work. But, if you forget to deactivate when you leave the house, you're hooped at the office! So, I gave up and upgraded to CS4, then CS5. Just need to upgrade to CS5 on the second work graphic station and I'm good to go! All I run at home now is Photoshop CS5, and it's a legit purchase upgraded from Photoshop Elements!