Moving the proprietary stuff out to a separate "extension pack" (that could easy be replaced with a third party open-source version) is really a nice step.
How so? The separate proprietary and open source versions have been around before the Oracle acquisition. Oracle hasn't done anything to make it more attractive to open source developers.
Rather than one binary blob, the extension pack uses a fairly well documented set of interfaces.
I took a quick look at the updated header files this morning and there is a _ton_ of API changes in VirtualBox 4.0. That coupled with the release just before the holidays start for me and a weeklong trip to Maui means that I can't give a reliable timeframe for pushing in support for 4.0 and testing it thoroughly.
The work is starting now, but I fear this is something which won't be available to Vagrant users until early 2011.
All in all though, I think these changes are for the better (especially the session related stuff).
(one hell of a restriction, but it doesn't have to require hacking) http://blogs.vmware.com/vmtn/2008/01/virtual-leopa-1.html