Once you buy the 3.0 firmware when it comes out this summer, during the upgrade, your previous firmware is completly erased. As a matter of fact, YOUR WHOLE iPod or iPhone WILL BE WIPED! (complete formatting is done)
iTunes first makes whole backup of your iPod/iPhone, then it wipes your iPod/iPhone clean, then installs the new firmware, then puts the backed-up data to your iPod/iPhone.
Basically, once you install 3.0 firmware, you can't go back! The previous 2.0 firmware is gone from your iPod/iPhone for good. It's like having new iPhone with 3.0 already installed.