The Team Foundation RC should be available for download sometime today, and Jeff Beehler provided a high-level description of what's changed. His first point is the most significant. There are many, many bug fixes in the RC compared to beta 3. For version control, merge has improved the most. It's quite a bit faster and more robust.
Rob Caron has posted information on upgrading from beta3 refresh to the RC, including the Team Foundation RC Installation Guide. Those with beta 3 installed will need to upgrade their servers to beta 3 refresh to then upgrade to the RC. Clients just need the new RC client installed, regardless of the beta 3 variant currently installed.