Originally Posted By: six_of_one
Once you convert, Mail, Contacts, and Calendar move to iCloud and are no longer available via MobileMe.

Mail is still accessible via the Mail app in OS X. Calendar via iCal appears to be inoperable on my laptop that has yet to be converted. Can't say regarding contacts. So using the OS, mail still works on Macs that haven't been converted =)

Haven't tried stuff via the actual MobileMe web interface ...

Mail is not accessible via the MobileMe web site after converting to iCloud. Of course, the desktop Mail.app is going to work whatever you do (until MobileMe is shuttered next June). After that, .mac/.me mail accounts will not work if you're not using iCloud. If you have other mail accounts that are not .mac/.me, they will continue to work as usual.

iCal will not work on your unconverted laptop if you are using the same Apple ID.