Nikon, Tokina, Tamron and Sigma
I've looked at a lot of 'general purpose' zooms, and here's my basic analysis:
Nikon 24-120 - Sounds good, but some the reviews are not very positive. It's pretty expensive, but not too bad.
Tokina 24-200 - Great range, but lots of talk of lack of sharpness.
Tamron 24-135 - Generally good reviews, and Tamron makes good lenses. The price is good too.
Sigma 24-135 - Good reviews, and f2.8 at 24mm. Unfortunately, no zoom lock, but other than that it looks really nice.
So, it looks as if the Sigma 24-135 is the winner. Also, at $300, it's a great price. In fact, the low price actually makes me suspicious, and is the only thing slowing me down.
There is one wrinkle ... Sigma just announced a new lens, 18-125 (f3.5). Now that sounds REALLY great. What a superb range for a D-SLR. But, I called Sigma customer service this morning, and they couldn't provide a ballpark on the price, and said that it was unlikely to be available until the end of May or beginning of June. So that probably means July or August.
So I buy the 24-135 now, or wait until (possibly) August, and get the 18-125 (after others have used it and posted reviews).