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You can't restore a 3GS to anything other than 3.1. So no 3.0.
GB, you are going to have to do this manually, so to speak, go into Cydia and uninstall each package. I'd start with the applications and then move to the MS-related stuff.
Don't forget to manually remove your apps from categories and the SBSettings dock/Poof.
Then go into User>Library>Preferences and clear those files out.
After that, using sFTP or a phone browser, look around for extra files you can delete.
Is going to be helkpful if you just copy your themes folder(s) over so when you reinstall WB, assuming you do, you can just drop them over and you have everything you need easy.