It seems virtualbox-bin ships with a version of rdesktop (probably with USB support patches). If you emerge with USE="rdesktop-vrdp" it gets installed, but only root has permissions to execute it. However, I found that in bug #300785 there was a patch for USB support added to rdesktop within portage. Depending on that is probably a better long term fix (and more Gentoo). Reproducible: Always
No longer applies to the 4.x.y series which is in tree