The trial version of Server 2012 RTM is now available from Microsoft. The TechNet Evaluation Center is a little confusing as it states "Download Windows Server 2012 Release Candidate (RC)" at the top of the page. However, comparing file names to the RC confirms that this is the RTM.
RC - 8400.0.WINMAIN_WIN8RC.120518-1423_X64FRE_SERVER_EN-US-HRC_SSS_X64FRE_EN-US_DV5
RTM - 9200.16384.WIN8_RTM.120725-1247_X64FRE_SERVER_EVAL_EN-US-HRM_SSS_X64FREE_EN-US_DV5
Download the Server 2012 RTM Trial here.
Microsoft are giving away a free ebook introducing Server 2012 which is worth a grab.
CBT Nuggets have released their First Look Series to Youtube for free too: http://www.youtube.com/playlist?list=PL282955A708FD70C0&feature=view_all
Azure
Microsoft Azure are providing a 90 day trial including a Server 2012 RTM virtual machine.
Signing up is quick if you already have a Microsoft account, but you will need to enter your credit card details. This made me a little on edge first but their policy seems pretty sound stating:
To protect you from accidentally incurring charges for usage beyond the included offer amount, we have introduced the Spending Limit feature. All new customers that sign up for the 90-day trial offer or one of our member offers (e.g., MSDN offer) will now, by default, have a Spending Limit of $0.00.
When your usage exhausts the monthly amounts included in your offer, we will disable your service for the remainder of that billing month, which includes removing any hosted services that you may have deployed. The data in your storage accounts and databases will be accessible in a read-only manner.
If you're not quite ready for Windows Server 2012 you can deploy any of the below instead.
Once signed up and signed in creating a virtual machine takes minutes.
Happy learning!