More info. http://www.kyngchaos.com/blog/2011/20110730_lion_first_pass
You can fix your Postgres installation with these commands in a Terminal:
sudo chown -R _postgres:_postgres /usr/local/pgsql-9.0/data
sudo chown -R _postgres:admin /usr/local/pgsql-9.0/var
sudo dscl . -delete /Groups/postgres
Then edit /Library/LaunchDaemons/org.postgresql.postgres.plist and change the UserName string to:
_postgres
Now reload the launch daemon: (or simply restart your Mac)
sudo launchctl unload /Library/LaunchDaemons/org.postgresql.postgres.plist
sudo launchctl load /Library/LaunchDaemons/org.postgresql.postgres.plist
Installers should all work, except that Postgres won't start. Just apply the above fix after installing and you should be good to go. I'll have a fixed installer out soon.