Some of you may be familiar with the "AT Warm Standby" feature that was included with TFS 2005 (aka Whidbey).
This feature is also available in TFS 2008 (aka Orcas) but with some slight changes.
Our guidance to people using this feature is to install/configure SharePoint and Reporting Services off of the AT (or move them, for existing installs). These products have their own availability features, and configuring both the primary and standby ATs to use the off-box WSS and RS means less things to do during a failover procedure.
This is different from Whidbey - back then, you were required to have SharePoint and RS installed and configured on both ATs.
To install TFS 2008 with an off-box RS instance, you'll need to modify your msproperty.ini. Sudhir has a blog post with the details. Note that this capability (off-box RS and AS) was not in Orcas Beta 2 but will be in the RTM version.