@OP: It most definitely is possible to boot from a USB flash drive. I do this at home, running FreeBSD 8-STABLE off a 2 GB Cruzer MicroDrive. It's connected to a USB2 port on a generic/el-cheapo P4 system. I've also done it with generic AMD Sempron systems, eeePC 701 and 1005H systems, and a handful of other random machines.
It requires USB Mass Storage support in the BIOS and the OS (which pretty much everything made in the last 3 years supports).