SJJ Embedded Micro Solutions have announced an XPE development kit that fully supports Windows XP Embedded SP2 FP 2007 (uncluding USB 2.0 Boot).
From the SJJ IDK Page:
The Industrial Development Kit (IDK) for Windows XPe allows OEMs to create dedicated applications using a small compact PC hardware from EMAC (PCM-9375 Single Board Computer). The kit supports Windows XP Embedded Service Pack 2 Feature Pack 2007 and specifically the new USB Boot 2.0 feature that allows the system to boot from a USB flash key. The IDK also features the Windows XP Embedded Supplemental Toolkit - Tools Release 2 to help create new components for your application.
This looks like a neat development kit if you are interested in working on Windows XP Embedded solutions.