Having upgraded to Lion Server on one of my boxes, I found that Postgres was indeed running. Or at least trying to run. It seems that the permissions weren't set properly on the conf file or data directories, so my log was filling up with "failure to start" messages as it tried to launch Postgres every 10 seconds. Even a "repair permissions" failed to solve the problem, and I had to manually go in and change some ownerships from the root user to _postgres.

Not sure if this is just my issue, but if you did upgrade to Server, you might want to check your system's log file.