I am having the same problem trying to get 8-port USB serial device attached to the XP host working on the Ubuntu guest (Ubuntu 10.04.3).
I am running VirtualBox 4.1.8 with extensions installed on the host.
The 8-port serial adapter appears to be set up as (internal) hub and eight individual FT232R serial interface chips.
When I start the guest (with the USB serial adapter attached to the host XP machine), virtualbox will usually grab 6 or 7 of the 8 ports, however, which ports are grabbed is inconsistent from one startup to the next. If I try to attach one of the orphan ports (right-click on the USB symbol at the bottom of the guest window to select one of the unfortunately de-selected USB devices), I get an error message "Failed to attach..." with E_INVALIDARG and "busy with a previous request".
This is really unfortunate misbehavior because it makes it difficult/impossible to deliver my solution if it behaves differently each time.
From searching/reading I see I am not alone in this issue, but I did want to add to this thread so an example of the misbehavior is shown on 4.1.8.