We are using an 11" Acer Aspire:
http://www.amazon.com/gp/product/B00MNOPS1C/
Once the junk (which it didn't really have much of) was removed, it runs the DS software without any issues. I think during a match the CPU commit went up to maybe 50%.
The battery lasts about 8 hours, there are no moving parts so you can toss it around and not really care, and it has an Ethernet port and several USB ports. It's even low cost enough that at the end of the season we can just pack the whole OI panel (it has custom buttons and the like) away with the robot so that it's ready to go for demo days.
In regards to serial (RS-232) ports, just use a USB<->RS-232 cable like this one:
http://www.amazon.com/TRENDnet-USB-R...dp/B0007T27H8/
I use them a lot for work, and they are perfect for flashing the Jags or interfacing with the old cRios.