The latest release of VMware has added experimental support for Ubuntu.
The erlease note says it:
VMware Workstation runs atop a host operating system, either Linux or Windows, and lets a computer run a larger variety of guest operating systems. The current product can run 64-bit host operating systems, but version 5.5 adds support for 64-bit Windows, Linux and Solaris.
In addition, the software now has support for FreeBSD 5.3 and experimental support for version 5.04 of Ubuntu Linux.