Using iCloud on Leopard with iPhone 4
If you migrate to iCloud now, you


get all the iCloud benefits on the iPhone
can still access your mail from your Mac (HT4864)
you should still be able to share calendars and contacts (see MacRumors but I haven't had a need to test this yet). This may actually be unsupported by Apple but if everything else fails you could always set up a Google Calendar for this.
will loose all the other MobileMe benefits (like bookmark, pref, keychain syncing, gallery, iDisk etc.)
will still be able to sync the content of your iPhone with your Mac