Now, courtesy of sp3dev @ XDA-Developers, a solution has been found... and i've taken his basic instructions and posted them below with full screenshots and the very latest driver files extracted from ActiveSync 4.5.
News page readers can find the 16-step (not as bad as it sounds!) walkthrough after the jump!
Before starting: Your device needs to be connected to the PC, and in BOOTLOADER mode.
Step 1 of 16: Extract this zip file to a memorable location on your machine (e.g. the desktop). This is the Activesync 4.5 RTM USB driver.
VistaRUU.zip 37.8KB 19664 downloads
Step 2 of 16: Run 'Windows Mobile Device Center', and click 'Connection Settings'.
Step 3 of 16: Uncheck the 'Allow USB Connections' box, and select 'OK'.
Step 4 of 16: Run 'Services' by clicking the Start Orb and typing 'Services', and ensure the 'Windows Mobile-based connectivity' and Windows Mobile 2003-based connectivity' services are started. If they are not, click the service name and press the 'play' button on the top bar.
Step 5 of 16: Load 'Device Manager' by clicking the Start Orb and typing 'Device Manager'. Expand the 'Mobile devices' section and click the 'HTC USB Sync' (or similarly named) item.
Step 6 of 16: Click the 'Action' menu and select 'Update Driver Software'.
Step 7 of 16: Click 'Browse my computer for driver software'.
Step 8 of 16: Click 'Let me pick from a list of device drivers on my computer'.
Step 9 of 16: Click 'Have Disk'.
Step 10 of 16: Click 'Browse', and browse to the location where you extracted the ZIP file in Step 1.
Step 11 of 16: Double click 'wceusbsh.inf'.
Step 12 of 16: Select 'HTC USB Sync' from the list, and click 'Next'.
Step 13 of 16: Wait while your driver is installed.
Step 14 of 16: Run 'Task Manager' by clicking the Start Orb and typing 'taskmgr'. Then find the 'WMDC.EXE' item, and click 'End Process'.
Step 15 of 16: Install your update! Everything should now run fine! You just need to bear in mind the final step when you want to start syncing normally again...
Step 16 of 16: In 'Windows Mobile Device Center', re-enable the 'allow USB connections' checkbox you disabled in Step 2.