No way. It is replacing the entire firmware. No way to tell who has what installed. It does however retain your preferences. My mail, weather, contacts, etc were all there. Ready to run. If you use the correct upgrade, from the Dev team all is great. i just restored my 3rd party apps. Only one I can not get to work is Make it Mine. It loads and works, but does not do anything. The iPod name does not change. I can live with that.
Customize and be tricked also, but I only used it to move my icons. Not necessary now.
Summerboard can work with some tweaks, but no backgrounds. Unless you like the special icons, I would wait for an update.
iTunes also recognizes my Touch has bee upgraded. One happy camper.