Heres the immediate confusion that could have been easily avoided: The main page at me.com/move seems to strongly suggest that if your computer is not yet upgraded to Lion, the main requirement for iCloud, you cant move to iCloud. The statement is that ALL devices must be compatible in order to "move".
But then, at http://support.apple.com/kb/HT4929
the first sentence is
"If you cannot update all of your devices to the iCloud system requirements, you can still move to iCloud, but iCloud services will work only on the devices that meet the system requirements".
So, if you want iCloud features on your OTHER devices, (most useful if you have more than one other device), you would move to iCloud (paradoxically -- in my case-- using a computer that will not (yet) be able to use iCloud), and reset your Mail settings as per the link above. You can still access some synced data on the non-Lion machine via a browser at iCloud.com.
From my read of HT4929, if instead you opted not to move, but only "keep using my email", (if you did not care about any other syncing), you might NOT have to change your Mail settings, (thats been my experience so far) or you may have to soon, when things actually shift (sometime after the supposed deadline) at the mail server-side, (as per Rogers comment above).
Thats how I see it.