Windows CS4 with a 64 bit OS installs both 32 and 64 bit versions and the 32 bit plugins work fine in 32 bit mode. Perhaps the same will apply to the Mac.
Actually, I have some favorite plugins and would want to update them anyhow for the new features.
One of the biggest reasons for me to upgrade now, while I can get CS4 and CS5 for the one price, is that having the later versions can save a lot of money down the road; now that Adobe limits the upgrades to the last few versions. Might as well upgrade rather than have to buy the whole thing over again - assuming one will be replacing computers and operating systems; at some point the older software won't transfer.
Life was so much less complicated before computers!