Originally posted by Mann Thanks for all the info and suggestions, guys. I just had a look at the Sigma 17-70, and I see it doesn't have the constant f/2.8 like the 24-70. That's kind of a turndown. I also had a look at the Sigma 24-60 f/2.8 EX ASP and the 28-70 f/2.8 EX DG. Do you have anything to tell me about those? If I'm going to buy such a lens, I'd like it to be as versatile as possible.
The 24-70/2.8 is considerably larger and more expensive than the 24-60/2.8 though some say the 24-60 is at least as good optically. I have one but tend to prefer my FA24-90/3.5-4.5. If you like larger cameras and you're shooting pro-style (read "big", "heavy") you'll probably want the battery grip as well.
Very few zooms are true 1:1 macro but offer relatively close focus capability which can be very convenient, especially when compared older lenses where minimum focus distance was a more frequent limitation.
Other good standard zooms are the the DA17-70/4 and DA16-45/4. Bear in mind that for many people 24mm isn't wide enough for their walkaround standard zoom. My latest acquisition of this sort is the DA17-70/4 with the idea that it's a pretty good choice for when I don't want to carry a bunch of lenses. If I need more speed or quality I can use primes. Pentax has a great collection of compact top-quality 'Limited' primes (15/4, 21/3.2, 31/1.8, 35/2.8 Macro, 40/2.8, 43/1.9, 70/2.4, 77/1.8).
If you're planning on getting a fast zoom you may also want to pick up the Pentax 18-55 kit lens for when you want something smaller & lighter (and less valuable?).