except for Palm Treos
Well, though
modest, I would suggest the 75x series
has been getting thinner and smaller (no antenna). The 5xx series (e.g. Gandolf) will also take this much further making device that are analogous to the Q/Dash/BJ devices.
So I think it holds there too. I think that Palm is more adherent to larger batteries than to slimness where possible for functional reasons.
As thin as devices like the Q are though, people often complain now that the ergonomics when typing are thrown off, since it is
too thin (it's a legitimate complaint often found in those forums). Hence rounded-backs tend to "feel better" but alas, those result in thicker devices.
Like I said, there are always trade offs in design.