We try to work in a 4 people team with 2 mac and 2 pc. After many problems to use the mac version, we just discover than we cannot open a file already opened on mac to read only. Same problem with the Xref: if someone works on a Xref file on mac, an other architect cannot see this Xref attached on an other file.
PS: we are using on pc autocad 2007 and on mac autocad 2011
I posted on this extensively in the thread AutoCAD for Mac/Mixed Computer Environment Mac & Windows
I created a tech support case for this as a part of that posting process, and tech support confirmed it was a problem that had been escalated to the programmers. As I read the docs of Autocad for Mac 2012, this is supposed to be fixed. I have installed 2012 and used it, but have not tested it in the office yet.
As for 2011, you have to resort to what we used to do in the "old" days when xref's first were invented - make copies and talk to each other to make sure you don't overwrite each other's work.