So, given that my second choice for a replacement handset would have been the Motorola Droid/Milestone, I was quite pleased to find http://www.androidonhtc.com which hosts a project for running Android on several of HTC's Windows Mobile devices (recent ones like the HD model's aren't supported yet).
It doesn't actually replace Windows Mobile - that remains as the primary OS that the device boots into. What it does is use a little utility called 'HaRET' which you run under windows which reboots the whole device into Android, similar to what loadlin did for DOS years ago - basically the running windows image is replaced by a Linux kernel and a copy of Android. Installation of Android is just a matter of dumping the files onto either the root of your memory card on an 'android' subdirectory thereof (Internal Storage on the Diamond), copying the appropriate device configuration file to 'default.txt' in the place you extracted to, and running HARET. The first time it starts up it takes a bit longer because it automatically creates a flat file to use as it's filesystem, but it's all automatic. Simple ...
Also if you choose to keep everything in an 'android' subdirectory then you can keep multiple versions on the card and rename whichever one you want when you're ready to start it. Great for testing things on different OS versions!
The first version I installed (which was the 24th January 2010 build, first on the download list at the time of writing) had a very bad problem where it was soaking up very large amounts of battery power. I couldn't figure out why since even after stopping everything I could (removing the connection to my google account, disabling wireless, disabling bluetooth, stopping location updating) it still didn't allow the device to run for more than 2-3 hours. I've now replaced it with a different build and it's working better.
 - I'm not sure why but I couldn't get any of the older ones didn't work. It didn't clear the Diamond's screen properly and just froze (A black line appeared on the left of the screen but that's all). Note that some earlier versions are designed to be installed into the root of the memory card/internal storage, not in an 'android' subdirectory. I'm hoping all future ones will be ok under 'android'
Once you get used to what all the buttons do, it works really well and is a great way of giving me an Android device for free, so congratulations to the team responsible for making this stuff available to us! It's not perfect, the camera, and some other things aren't yet supported but most of the other bits work. I'm currently booting into Android 2.0.1 from February 2nd, 2010, but various versions are available and the fact that everything's in its own directory on the memory card means that backing up and trying a new version is easily done by just renaming the old directory, and restoring it if required.
Now do I want to start getting into Android development, or will I just leave it as a toy?