I swear the timing with geoapps's previous post is totally a coincidence!
I have started developing a little program to automagically update ClamWin Portable's virus definition files. It's not been extensively battle-hardened yet, so please post your comments/bugs reports/etc or send me email. But I have been using it for the last few days and haven't had any problems.
You can download it from http://www.clamup.co.nr/.