Bil Simser asks the question "Do SharePoint Developers Want a Developer Version of SharePoint?", and I've got to say YES. Yes, Sharepoint desperately needs a version that a developer can use on a desktop OS. As Bil mentions, Biztalk has this, why not Sharepoint? Developing on a Virtual Image (VPC or VMWare) so you can run Win2k3 Server is a pain, and not always possible (I know of some financial firms in the area that do not allow it). If you try really hard, yes you can do some development using a desktop OS and use remote debugging, but all the development tools I've used seem to require Sharepoint to be installed along with Visual Studio.
I was sort of, kind of, hoping that once the process of installing MOSS or WSS 3 on Windows Server 2008 was smoothed out, there would be some sort of announcement that WSS installed on Vista would be possible (and supported). My thought process was, since Server 2k8 is running IIS 7, and Vista is running IIS 7, that it would be possible to install WSS 3 on Vista. Now, I've never tried it, and I'm not sure if there are licensing issues, so I'm not going to try it. It seems as though Andrew Connell had the same idea. But if anyone is listening, yes, I'd really like to have a version of Sharepoint that can be installed on my desktop OS (aka Vista). Until that time, I'll make do with a combination of remote debugging and/or developing on virtual machines.
As always, these are just my opinions and I haven't heard officially or unofficially of any plans to have a developer edition of Sharepoint (or even the inverse, that there was no plans).