There's a new whitepaper on MSDN that I've authored called Building Servicable Devices. It's been in the works for a while - I wrote the bulk of it a few months back, but then ran out of bandwidth with all the XPE SP2 work I was doing. By the time someone got traction on it, it was July.
Anyway, the paper is there and covers some high level topics on how to think about servicing XPE devices. There are links to key Windows technologies that can help, and there's talk that a more in depth paper is coming as well - more on that as it happens.
Let me know if any reads the whitepaper (or this blog for that matter).