Microsoft quietly RTM'd SCCM 2012 over the weekend. At the moment it only looks like it's available for Volume Licensing customers. Presumably the official release date will be April 17th 2012, the morning Brad Anderson does the Keynote at MMS 2012.
If you're having trouble finding it in the download center, that's probably because it is not listed under Configuration Manager. Searching for Configuration manager will only find SCCM 2007. SCCM 2012 can be found listed under the System Center 2012 Standard and System Center 2012 Datacenter suites.
Thankfully the suites are logically split into collections of ISO files. The Standard Configuration Manager and Endpoint protection ISO is 1.67GB in size.
If you've installed Configuration Manager before you'll recognise the installer. Let the migration begin!