Confirmation that the Maplin USB to Serial (DB-9) two port adapter (Order Code N04GF) is detected by Fedora 12 and works perfectly. The packaging also has the product code BF-816 which reveals that the adapter is actually a BAFO Technologies product, which uses the MosChip MCS7720 controller (driver downloads here).
Kernel output below:
usb 5-2: new full speed USB device using ohci_hcd and address 3
usb 5-2: New USB device found, idVendor=9710, idProduct=7720
usb 5-2: New USB device strings: Mfr=0, Product=0, SerialNumber=0
usb 5-2: configuration #1 chosen from 1 choice
USB Serial support registered for Moschip 2 port adapter
mos7720: 1.0.0.4F:Moschip USB Serial Driver
moschip7720 5-2:1.0: Moschip 2 port adapter converter detected
usb 5-2: Moschip 2 port adapter converter now attached to ttyUSB0
usb 5-2: Moschip 2 port adapter converter now attached to ttyUSB1
usbcore: registered new interface driver moschip7720
Currently on sale at Maplin for £15 (until 26/01/2010)