Running gui apps as other user

[Todd Slater]

My wife and I share a machine and sometimes one of us wants to run an app
without logging the other person out. Normally we'd do "su - user" and
type the command to run the app, but Ubuntu (Hoary) doesn't let us; we
always get the error "cannot open display".

I read something about xhost +127.0.0.1 but I did this and got the same
error.

Ideas?

Thanks,
Todd