This was speculated about early on, but never came about.
What a shame. I remember their statement about how important it was for join forces to prevent Xara Xtreme from being killed by Illustrator.
The piece of most interest in sharing - the renderer - was never open sourced.
Reading http://en.wikipedia.org/wiki/Xara_Xtreme
I got the impression that it would have to replaced with Cairo no matter what, due to license issues and that Xara's render only compiled on i386?
Isn't Cairo superior?
The opened portions are mainly the UI, which unfortunately was implemented using wxWindows rather than gtk, so is not directly transferrable.
I see.
That said, if anyone wished to do some mining in the source, perhaps there are useful ideas that could be borrowed?
If you know of a manual for Xara Xtreme, I would love to read it, and make list of features that it have that Inkscape doesn't.