Sorry, I misread your original post. The SD slot on the side of the netbook is the boot-able one. It's the internal one (behind the battery, microSD) that can't be booted from.
There's no reason you can't boot from battery slot either; U-Boot will check both slots (battery slot first) but there are a few issues with the microSD slot with regards to power supply (it only does 3.15V and not the full 3.3V) which means some bigger cards (8GB, 16GB..) just don't work.
The real problem is going to definitely be finding a card that will "work" within U-Boot, but not a fundamental impossibility.
Matt Sealey, Genesi USA Inc.
Product Development Analyst