Yes, Guest additions are added.

More on that issue:

Here's how my USB is set:

br.geocities.com/marcelotodaro/vb_usb_filters.jpg

Following someone's suggestion I removed all but one filters and left it
unchecked:

br.geocities.com/.../vb_usb_filters_2.jpg

Whether or not the box is active results are the same after restarting
the virtual machine: no USB device available.

But hey! I found something interesting. I removed the last filter, then
I disconnected the webcam, connected a pendrive and restarted the
virtual machine. This is what happened:

br.geocities.com/marcelotodaro/vb_usb_devices.jpg

Note that VirtualBox detects USB devices connected to the hardware but,
no matter there are or are not filters for them, it doesn't make them
available.

Does that say anything to you? (I wish it did to me...)