The TFS Rangers team has just published a guide to help you get ready to upgrade from TFS 2008 to 2010.

This guide covers scenarios which may be encountered during and after the upgrade process. It provides examples of most common and potential issues. It covers scenarios related to general Upgrade Process, Work Item Templates, Reports, and Enterprise TFS Management (ETM). Although this guide refers to TFS 2008 scenarios, almost all the concepts apply also to TFS 2005.
Visual Studio ALM Rangers
This guidance is created by the Rangers who have the mission to provide out of band solutions for missing features or guidance. This content was created with support from Microsoft Product Group, Microsoft Most Valued Professionals (MVPs) and technical specialists from technology communities around the globe, giving you a real-world view from the field, where the technology has been tested and used.
What is in the package?
A single document with the following contents:
1 Introduction
2 Upgrade Process
3 Scenarios
3.1 Upgrading Projects from Multiple TFS 2008 servers into one TFS 2010 server
3.2 Upgrading severs when SQL Mirroring is enabled
3.3 Recovering system if upgrade fails midway
3.4 Updating Team Project Portal for an existing Team project
3.5 Splitting Team Project Collection into Multiple Collections
3.6 When I move a Team Project Collection, how do I move the reports?
3.7 Bringing Workgroup Machine inside Domain
4 Frequently Asked Questions
4.1 Can I use a TFS2008 Process Template to create team projects in TFS 2010
4.2 How can I enable Agile Workbooks in upgraded Projects
4.2.1 Enabling the Product Backlog Workbook
4.2.2 Enabling the Iteration Backlog Workbook
4.3 How can I enable Test Case Management in upgraded project
4.4 How can I enable Branch Visualization in upgrade projects
4.5 How can I enable Lab Management in upgrade projects
4.6 What is WIT Admin Tool
4.7 What happens to my custom reports created in TFS2008 post upgrade?
4.8 Will my old TFS2008 reports work post upgrade?
4.9 Can I add a new Database to my existing TFS 2010 farm?
4.10 Error bringing cloned Team Project Collection online
4.11 Move Team Project Collection Database from one Database server to another Database Server
4.12 How to enable the TFS reports if the WSS server is upgraded to MOSS server?
4.13 How to Move Team Projects from one Team Project Collection to another
5 References

The guide is published on CodePlex here: http://vs2010upgradeguide.codeplex.com/wikipage

Note that we will have specific instructions on how to upgrade from 2010 Beta 2 to the RTM version, this will be published once RTM is released along with all the rest of the RTM documentation.