ptosh: It's just because we already have the program we need in OpenUSB. If OpenUSB does not work on the Sheeva, we will be forced to rewrite it for libUSB.
libUSB 0.1, the one on the repositories, does not admit callbacks. libUSB 1.0 does, so I guess we will install that one.
However, I just can't find a reason why OpenUSB would not work on Sheeva!