It is possible, though. Mach in itself is not a complete kernel, at
least not in the traditional sense (i.e. it is a microkernel :-). You
still need file system drivers, networking stacks, process
management,...
In the past, people always put an existing operating system on top of
Mach. CMU themselves always used BSD, AFAIK. Because of the BSD
copyright problems they could never release that to the
public. Another port, Lites, put 4.4Lite on top of Mach 3. The OSF RI
has been using both OSF/1 and Linux. All these systems are known as
single-server, because the traditional OS features all reside in on Mach
task.
There is only one true multi-server system on top of Mach, the Hurd.
To me, it seems very possible that Rhapsody is 4.4BSD based.
Regards,
Martin
P.S. Don't tell me this is not about Linux. I know it is not :-)