The link quoted above assumes you are migrating with Lion.
If you do not migrate before the end of June you will lose your email account. You can migrate your MobileMe account to iCloud at http://me.com/move even with Leopard - when asked to confirm that you have Lion on your Mac, just lie and say 'yes'.
If you have MobileMe mail set up to collect mail from external POP accounts you should cancel this before migrating, or you may find it still working in iCloud (where it isn't supposed to) and with no way of stopping it. You may find the MobileMe email settings will continue to work for a time after migrating but it will stop eventually so you should not rely on this. You should delete your MobileMe email account from Mail.
You can then set up Mail manually in Leopard to access iCloud email; the process is described here:
However you will not be able to access any of iCloud's other facilities including calendar & contacts syncing. You can access the iCloud web page at http://icloud.com but your calendars and contacts will not be there: you can access your email there.
There are a couple of third-party solutions which may be of interest.
- BusyCal is an iCal-like calendar application with extra facilities: it can sync with the iCloud Calendar while running on Leopard, Snow Leopard or Lion. A single user licence (two machines allowed) is $49.99.
- Soho Organizer can sync Calendars and Contacts with iCloud on Leopard, Snow Leopard and Lion. A single user licence (multiple machines allowed) is $99.99.
Thanks, Roger, for some very useful advice. I've been a little put out with Apple leaving loyal customers out in the cold (I've been using their online service since it was dot mac). I don't mind so much having to upgrade my MacBookPro from Leopard to SL and then Lion (I bought the SL disk, though finding the time when I'm not in the middle of a critical project has been a problem), but I've been wondering what I might still be able to do with my G5 tower (PPC) when I'm forced to go to iCloud. It's an excellent machine despite its age and if I can get both email and a decent calendar sharing set up for it, it might get my daughter through middle school. Of course, we might also just switch over to Google calendar and gmail and other services if that seems simpler; it's certainly cheaper.