Hi srwsol,
the last thing was that I uploaded different logfiles (ESXi 5.0-914586, ESXi 5.1-914609 and ESXi 5.1-1021289) and complete guidance for reproduction for the third time to simply prove, that USB Controller Passthrough worked under ESXi 5.0 and doesn't work with ESXi 5.1 on the same hardware (HP DL360 G7).
According to the logs of the ESXi 5.1-914609 the BIOS seems to tell ESXi that Interrupt Remapping is disabled, and that was why the USB Hostcontroller wasn't put into Passthrough Mode during reboot. But there's no separate option in this BIOS and VT-d was enabled all the time.
Unfortunately since then only silence, no responses to the uploaded logfiles or following questions about the status... Wait since approx. 1 week after the last patch release now...