Originally posted by LIJ I guess a constant 2.8 is out of the question but I wouldn't mind a constant 4 or a 2.8-4. (a constant 4 would be very close to the DA 17-70 which seems somewhat weather sealed so my guess is that it would be a 2.8-4 to offer something "more" than the 17-70).
Considering that offerings over that range are often slower (see Canon's 17-85 f4-f5,6) I wouldn't hold my breath for a 16-85 f2,8-f4. A f3,4-f5,6 is much more likely, and stretching that f3,5 to f4 wouldn't surprise me at all. Otherwise, you can expect a really large lens, and very expensive too.
Remember that even the 17-70 is "only" f4.
I see that lens more like a pro-grade equivalent to the 18-135, not a replacement for the 16-50.