Given how memory prices fluctuate, I'm not sure if including onboard flash memory/storage is something manfucturers want to get into, especially for a Windows-only feature.
I actually read of people doing this (using flash drive as cache) a few years ago. I believe you can actually do this right now with any Windows PC which allows you to select which drive to use for cache? I think I'll try this out tonight as I have a spare 2GB flash drive from those GCO/buy.com deals.