The beagleboard is somewhat painful. It has a cortex-a8 cpu, which is quite a bit more complex than older arms. The lack of built-in ethernet means that getting USB going is vital, but the EHCI registers provoke access exceptions and the OTG registers are like no USB interface we've ever seen before.
A beagleboard with built-in ethernet is available from www.igep-platform.com and looks like a better bet (assuming that they include programming documentation for their ethernet controller). If you aren't trying to build a terminal, the marvell sheevaplug works well: $100 in quantity one, 1.2GHz ARM926EJ-S, 512MB of ram, some flash, built-in gigabit ethernet, (OTG) USB with a superset of the EHCI registers (so not completely hopeless).
