Anyone running Virtuozzo or OpenVZ (the opensource betagrounds for Virtuozzo) is most likely overselling. Xen doesn't allow for the overselling of Ram, but disk and cpu can still be oversold.
Edit:
Here are some examples;
A few reputable Virtuozzo hosts that oversell:
Knownhost
SolarVPS
PowerVPS
TekTonic
Some Xen hosts that can't oversell Ram (compare resource prices
Slicehost
VPSland
RimuHosting
In my opinion, any VZ/OpenVZ hosts that don't oversell are kidding themselves.
There is no reason not to oversell at least to make up for some virtualization overhead. When done properly an oversold host node should run without problem.