I keep two (or more) versions of PostgreSQL installed on my laptop that I switch between depending on what project I’m working on.  In Snow Leopard, this involved setting the path for postgres to whatever version I was using.  In Lion however, it is easier to switch the home directory for postgres altogether; leading to an alias containing this command:

dscl localhost -change /Local/Default/Users/postgres NFSHomeDirectory `dscl localhost -read /Local/Default/Users/postgres NFSHomeDirectory | grep ‘/.*’ -o` /opt/local/var/db/postgresql91

That’s one command, and likely the longest I’ve personally written!