The official launch of Windows Server 2003 R2 (Release 2) took place earlier this week (Tuesday 6th December) - we'd planned to make the software available for download immediately available though unfortunately there was a short delay in getting the RTM (Release To Manufacture) code online.
If you'd like to find out what's in R2 then the best place to start is by reading the Reviewer's Guide - it's available here together with a whole host of other information. Personally the most interesting new feature is the ability to federate your identity system whereby you can enable a trusted business partner to provision access to a subset our your Active Directory resources and vica versa.
You can now download an evaluation version of Windows Server 2003 R2 from here - it's important to note that it's against the terms of the usage agreement to implement this software in production hence the term "evaluation" and the code is time latched such that it will cease working after 180 days. You will be able to get hold of the official release in the Spring (up to 90 days post launch is the expected timeframe) at which point you will be able to use it in production.
To access the evaluation software (which you can download or request on CD) you'll require a valid Microsoft Passport. In addition you'll have to register - bizarrely enough you'll be prompted to enter both a billing address and a deliver address even if you're requesting a software download!
Next you receive the option to request either the "Windows Server 2003 R2 32-bit x86 (180 day trial)" or the "Windows Server 2003 R2 x64 (180 day trial)" after which you can request either a download OR CD. Proceed to the "check out" where you have to accept the terms and conditions - i.e. it's eval code. The outcome of all this is that you'll receive an email detailing where to download the software from together with your license.
You have to download two ISO images - the first is an evaluation version of Windows Server 2003 SP1 (it's approx 566 Mb), the second is R2 (it's approx 120Mb) - you can't install the eval version of R2 on a non-eval version of Windows Server 2003. Burn both ISOs onto CD and you're ready to go once you've accessed the product key from your email.
The software is also on the MSDN Subscriber downloads site.