I have had the problem on my ubuntu 7.10 install with the latest vmware workstation release. The virtual system was running a CactiEasy install, which is the cornerstone of my network trendanalysis platform.
The problem does occur, when the ubuntu system was under high network i/o (>50Mbit/s). It simply disconnected the vmware from the bridging, the cacti install was only able to reach the ubuntu system and itself, but no other system. Not a good thing for a network monitoring system.
After the upgrade to 8.04.2 LTS of ubuntu this problem vanished. 🙂