Is Eclipse really "just" an IDE - or is it a tool's platform which can be an IDE ?
Maybe there could be some API cooperation or whatever.
When Eclipse 3.0 comes out the "Rich Client Platform" project will be done and Eclipse will truly be able to be anything. Anything with an SWT workspace window. Another big feature of Eclipse 3.0 will be the ability to place SWING widgets on SWT windows. That should be huge for people trying to migrate their tools quickly.
Sun's big sticking point (outside of pride?), as I understand, is all the work and extensions they put in NetBeans for J2EE and only they know how hard it would be to migrate. Last I heard they declined the invitation, but then opened up talks again soon after.
I think they should join, but it will either take away resources from their NetBeans team or force them to add new people ... oh I dunno. I'd like to see them join.