While trawling through the latest 2008 Books Online this morning to answer a question, I noticed a new section that I hadn't seen before, which explains in detail how to perform a rolling upgrade with database mirroring. The link to the MSDN page is http://msdn.microsoft.com/en-us/library/bb677181.aspx and below I've linked to the flowchart from that page.  

Books Online also has a lot of other "how-to" topics around database mirroring – here are some links:

Kevin Cox of the SQLCAT team also just blogged about a customer upgrade from 2005 to 2008 and some of the issues they faced. Btw – if you're not subscribed to their blog, you definitely should be – lots of cool stuff.

And now the bug. In 2008 RTM, if your database contains full-text then mirroring will not work when you perform a rolling upgrade. This is explained in KB 956017 (with a trace-flag workaround) and KB 957816, which points at 2008 RTM CU1 that has the fix in.