I wanted to get this into the tips and tricks class at AU but ran out of time...I had a client that was using Revit MEP 2009 and creating multiple projects for each discipline, due to the size of the project they were working on. After goofing around with one of the projects, we found out that you can copy and paste spaces from one room to another - as long as the linked file is set to be room bounding, and as long as you stay on the same level. If the linked file has spaces, it will pick up the room geometry once you select finish from the options bar.
So, we tried it between two different projects using the same linked background - and it worked! Again, it only works level to level, and we couldn't get multi-level spaces to work, but if you didn't create your spaces before breaking a project up into smaller projects, this is a fairly quick way to set one up and then use it other files.
Try it out and let me know what you think -
thanks - David B.