I believe I'm going to go for the 18-135 for now, pocket the savings on the 16-50, and hope that Pentax comes out with an improved DA* 16-50 F2.8 II or similar in the next couple years. It just seems a bit too risky of getting a bad one or losing the SDM motor at some point, not acceptable to me at that price point.
According to the limited number of reviews on the 18-135 that I have found so far, it is a worthwhile upgrade over the kit lens in both reach and IQ. Since the K-5 has really good high ISO performance, the speed of the variable aperture is less of an issue. I guess I'm still stuck in the film mindset that I *need* an F2.8 lens, because I used to shoot a lot of Velvia at ISO 40