What's your definition of portable?
If you're after something bigger than a hand-held scanner, I'd recommend something like a
CanoScan Lide 200. I just picked one up myself the other week to replace my old Lide 60 (That old thing just REFUSED to work with 64-bit Windows 7... Grrr).
It's a flatbed scanner, but very light and host-powered (i.e. no A/C power needed, voltage comes through the USB cable!). Definitely something you could get away trundling around with in a small carry-on suitcase.
We have them for about $160 AUD here in Australia.