A bit of a follow up for anyone who's interested in running Live Writer on 64 bit Windows, and didn't read the comments to my previous post.
Charles' explanation as to why the current install doesn't work with 64 bit makes a lot more sense:
We're sorry about the fact that Writer stopped supporting 64 bit Windows. You're right that this is just temporary. Our installer had a little hiccup but it will be resolved in our next release.
Once again, our apologies!
So all you 64bit people can look forward to being back and writing with Live Writer in the next release.
For those of you running Vista 64 bit, you can get a modified MSI here.
Thanks to Charles and Blair for the respective pieces of info!
EDIT: It seems the modified MSI linked above actually works fine for XP 64 bit too. When I ran it, the install completed silently, which led me to think it had failed.. by which point I had to run out the door to catch a train. I now notice that Live Writer did in fact install successfully!