I guess so. All I know is I looked into it a few weeks back and there is a load of stuff about how dual booting Vista on an external USB drive doesn't work.
And yeah, thats one of the links, of many, that I read. If you notice you get a lot of stuff about it isn't supposed to work, and possible workarounds, with one guy getting it to work after major hassle but still getting cock ups such as freezes.
Personally for the small price of an internal hard drive (although I do have quite a few external ones laying about) then I would rather do that than muck around with a potentially unstable OS. Just remember to also order the cables though. Mine didn't come with any and it turns out my bleeding edge rig (at the time) only has one SATA2 power connector and data cable. And the drive didn't come with any either. Doh.
Cheers
I am 99% probably lying in bed right now... so don't blame me for crappy typing
Current fave quote : "She was like a candle in the wind.... unreliable...."