Snow Leopard with iMac and iCloud?

Yes, by upgrading to Lion.

Upgrading to Lion

If your computer does not meet the requirements to install Mountain Lion, it may still meet the requirements to install Lion.

You can purchase Lion by contacting Customer Service: Contacting Apple for support and service - this includes international calling numbers. The cost is $19.99 (as it was before) plus tax. It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.

Lion System Requirements

1. Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7,

or Xeon processor

2. 2GB of memory

3. OS X v10.6.6 or later (v10.6.8 recommended)

4. 7GB of available space

5. Some features require an Apple ID; terms apply.

On Snow Leopard, you can only use iCloud mail, but you must have logged in iCloud on a supported device before > http://support.apple.com/kb/HT5330

Also, see if your Mac is compatible with OS X Lion > http://support.apple.com/kb/SP629 If it's compatible, call Apple to purchase Lion > http://support.apple.com/kb/HE57 With Lion, you can use iCloud without any problem