As soon as you're ready, the way to do it is to use the CS3 uninstaller which you'll find in Applications/Utilities/Adobe Installers.
Dave
Where you will have issues is if you simply drag CS3 to the trash.
Neil
Jens
NOTE: please make a copy of the 'Adobe Unit Types.osax' found Macintosh HD/Library/ScriptingAdditions/Adobe Unit Types.osax
At home did this without making a backup of the 'Adobe Unit Types.osax', so when I launched Photoshop CS4 it suffered from the "Error: "One or more files in the Adobe Application Support folder.."
At work I did this with making a copy of 'Adobe Unit Types.osax', place it back after the uninstall of CS3... and presto so far so good.
for the 'Error: "One or more files in the Adobe Application Support folder... are missing' when you start Photoshop CS4 (Mac OS)" read here:
<http://kb.adobe.com/selfservice/viewContent.do?externalId=kb407726>
sorry if this has already been posted, I could not see this anywhere.