Running multiple versions

I'd like to keep my current installation (8.3.3) alive and running while
installing and running 9.1.2 on the same server.  Can I do this using
only the existing postgres superuser account?  I'd want to create two
different initdb locations, and run the versions on different ports, of
course, but it seems like the superuser's LD_LIBRARY_PATH would be an
issue.  Whichever .../lib dir it points to, would be the only effective
one, and the "other" version's programs wouldn't work.  Is there a way
around this?  Or am I thinking about it all wrong?

nishad