Curious to see how many people upgrade vs clean install to OS 3.0. 
Edit - Since so many people asked how to clean install, here is step by step.
1) Download the OS 3.0.
2) Sync your iPhone.
3) Set it up as a new phone when asked.
3a) If there is no option to "set it up as a new phone," hit "Restore."
Now your iPhone is back to factory settings.
4) Go to "Info" tab on iTunes. (Between Summary and Ringtones tab)
5) Select your method to sync your contacts, calendar, mail, etc.
6) Hit "Sync" on lower right corner.
Now you are good to go.
Edit - Since so many people asked how to clean install, here is step by step.
1) Download the OS 3.0.
2) Sync your iPhone.
3) Set it up as a new phone when asked.
3a) If there is no option to "set it up as a new phone," hit "Restore."
Now your iPhone is back to factory settings.
4) Go to "Info" tab on iTunes. (Between Summary and Ringtones tab)
5) Select your method to sync your contacts, calendar, mail, etc.
6) Hit "Sync" on lower right corner.
Now you are good to go.