It isn't really a matter of having them running at the same time. Depending on how the EULA is written you can only have the current version only. Implying that having previous versions concurrently installed is not allowed. Think of it this way; If you get the upgrade then you have to get rid of the old stuff.
But you will have to read the EULA to know for sure.
We run the same type of setup as dabob so we looked this up a while ago... just in case
According to "Section 5: Updates" in the CS4 EULA posted on the Adobe website, they do allow you to continue to use older versions on a machine after you have upgraded. I think that this was to allow print providers to keep processing files from older versions.
"After you install such update or upgrade, you may continue to use any such previous version in accordance with its end-user license agreement only if (a) the upgrade or update and all previous versions are installed on the same Computer, (b) the previous versions or copies thereof are not transferred to another party or device unless all copies of the update or upgrade are also transferred to such party or device and (c) you acknowledge that any obligation Adobe may have to support the previous version(s) may end upon the availability of the upgrade or update."
Quark originally disallowed this, but after some user complaints they followed suit and changed their licensing as well to match Adobe's policy. From Section 10 of their EULA...
"Upon Installation of the Update, End User may continue to use the previous version provided that the Update and the previous version are installed on the same computers, subject to the limitation set forth in Section 1 hereof. Upon installation of the Update, End User shall not transfer the previous version to a third party."
If you're tired and need help getting to sleep you can get the Adobe EULAs here
Adobe - Adobe product license agreements
and Quark's here
Quark: Read the Quark Global Software License Agreement
Of course these are general EULAs for each product. It's possible that other specific licensing situations could be different.
Shawn