Nothing catestrophic will happen, in the worst case it may make some of the other extensions that did work on cs5 stop working because of overwriting shared files that were updated to work with cs5.
Having said that some basic functionality will likely work, just in some cases you can get things that may not work properly or even errors are possible. In the worst case you could still uninstall and reinstall the CS5 versions and they should begin to work again. We are just making small tweaks to account for any changes and testing all interfaces to make sure we haven't missed anything. Our entire extension suite will be fully functional and tested on CS5 in the next couple of weeks.
I'd suggest to stick to the CS5 approved list for now.
