Arthur,
For many plug-ins, yes, you can. However some may still require being reinstalled if they put any sort of information into the Windows registry. Also if you leave CS and CS2 co-installed, it would be advisable to put all 3rd-party plugins into a separate plug-ins folder lying outside the root installation paths of either version of Photoshop. This has the advantage of allowing a secondary plug-ins folder to be pointed to from the PS preferences and avoid the hassle and wasted space of duplicate installations. Also, by ensuring that you copy only the 3rd-party plugins and leave the native PS plug-ins as is, you reduce the risk of introducing errors caused by one version of PS using the plug-in from another version.
Regards,
Daryl