Putting multiple motherboards in 4U case

I have found that a 4U co-lo deals are considerably more cost effective
than other plans, but are restricted to 1 server. Our company co-lo's
about 75 low cost servers at a time, and are proud to say they are
cheap bastards. They want us to test putting several motherboards and
a switch into 1 4U server case. Because these servers are Xen based on
Gentoo, there is very little concern for individual server failure as
redundancy has been taken into consideration.

Putting cooling/power issues aside, which lm_sensors will help us test
and manage, has anyone else tried this? Any problems we can expect as
we go down this road??