Hi David,
sorry for taking so long to reply, I've been away gigging rather a lot of the last couple of months...
Bill uses, I think, an Eventide Harmoniser for a lot of his effects, so certainly has access to all manner of amazing pitch changing sounds, so I would guess that at least part of the time he's using an octave effect. It's a great sound, and one that's used by a heck of a lot of bassists, so much so that Ashdown saw fit to build on into their amp heads and combos (and a damn fine octaver it is too...)
hope that helps - check out Bill's website (can't remember the URL, but something about bass and buddha I seem to recall...)
cheers
Steve
www.steve-lawson.co.uk