The new Virtualbox 4.0.0 r69151 "OSE" has direct support for USB1.1 compared to earlier versions of Virtualbox which needed the "puel" version to have any support for USB either 1.1 or 2.0. Using a extension pack with a PEUL license will add support for USB 2.0 (ehci).
Since its USB implementation is now opensource, maybe it helps you trouble shoot why it boots incredibly slow like the earlier versions
http://forum.plop.at/index.php/topic,200.0.html ?
