I use OS X 10.6.3.
I installed the OSX PL2303 diver.
VirtualBox causes my computer to freeze every time I try to use the usb serial converter.
If I uninstall the OSX PL2303 driver, virtual box runs fine, and I can even use the USB serial converter running Ubuntu on VirtualBox.
I have seen this with the OSX PL2303 driver from SourceForge, and also with the driver that is patched to run on 64bits for OSX Snow Leopard.