Well, I can say for my own purposes, I can see a bit of a difference in a 2-pass, and the file remains the same (Batman Begins is ~1.8gig after conversion), so I'll probablly start going with 2-pass from here out, as I'm not generally crunched for time, and can let it encode overnight If I'm really lazy.

Thanks for pointing out exactly what 2-pass does, folks.