SQL Server 2008 R2 has hit RTM and the downloads for the Express edition and the Datacenter evaluation are up. The product page has not been updated as of the time of this post. It looks like both x86 and x64 environments are supported for all editions. Probably most notable is that the free Express edition increased the maximum database size to 10 GB (up from 4 GB in previous releases). The 1 processor and 1 GB of memory usage limits are still in place and SQL Server Agent is still absent in the Express editions. Also interesting that they didn't include the updated Express edition in Visual Studio 2010.
Note: As of the time of this post, the Release Notes and Readme links at the following download pages still point to the late-2009 CTP documents.
SQL Server 2008 R2 Express
SQL Server 2008 R2 Express with Management Tools
SQL Server 2008 R2 Express with Advanced Services
SQL Server 2008 R2 Datacenter Edition 180-day Trial (for IT Pros)
SQL Server 2008 R2 Datacenter Edition 180-day Trial (for Developers)
The last 2 files are identical, but you get different follow-up emails if you register.