Thanks - it actually didn't have to be modal (just a pop-up help/hint)
so I ended up just using Effect.Appear and Effect.Fade (which were
already available) on a custom DIV. Should suffice for now...
You could have a look at lightbox variants, I haven't had too in-depth
look though I've worked on a project using lightbox in rails and it
> I am looking to create a simple dialog window like this on a page: