In a word, don't.

iOS4 on iPhone 3G from Josh Russell on Vimeo.

• OMG slow!
• Third party apps don't work after a restart (this is huge, be careful not to let your battery run out)
• But you do get folders!

(yes ok, you do get some good stuff, multiple exchange accounts, syncing of notes to gmail AT LAST!, zooming camera.. but overall i'm not sure it's worth it until Apple release a fix)

I'm currently relying on web apps for everything at the moment. Another example for us web fans to use in the argument against native apps.