Thanks for your reply.
The WM5 (Acer n310) device does have a USB Host.
I downloaded the FTDI driver from their website and followed their instructions. Edited the inf file to the correct ID etc. then...
copied the driver and inf file to WM5 Windows folder.
Plug in the Arduino (tried Uno and 1280 Mega) and a dialog asks which driver.
I type the driver name (ftdi_ser.dll) as instructed.
That was accepted.
No serial port is created but the USB Host info in System Info gives the correct IDs and says no device connected, with both Arduinos.
I checked the registry entries as per...
http://www.hjgode.de/wp/2009/10/22/windows-mobile-install-ftdi_ser-serial-to-usb-adapter/
The only missing is as follows, all others were correct but used "COM" instead of "SER" but I tried both anyway.
;[HKEY_LOCAL_MACHINE\Drivers\Active\48]
;"FullName"="SER0"
;"Hnd"=dword:005A1A80
;"Key"="Drivers\USB\ClientDrivers\FTDI_DEVICE"
;"Name"="SER0:"