Well, I also recently puchased the Pentax K100D and I love it. However, I purchased mine with the kit 18-55mm lens, it is a nice walk around lens and lets me get as close as I currently need. I also have my mom's old 50mm 1:2. The digital crop factor turns it into something like a 70mm. It is nice for portraits and I have taken a few photos of my animals. In the future I'd like to get the 50-200mm Pentax.
My recomendation is probably the 18-55; it suits my needs. I also visit a forum dedicated to Pentax (d)SLR users. You can visit it
here. They'll be able to help you I'm sure! You could also visit this site
here, it lists all the Pentax made lenses that are built specifically for their digital bodies. It has listed a 16-45 which "features a 3X zoom ratio with focal lengths covering ultra-wide angle to normal ranges" and has a listed price of $549 US. Also, check out eBay, the camera is compatible with any Pentax K-Mount lens ever made (as well as the other mounts with adaptors), you may be able to find a old used lens on there cheap that suits your fancy (but won't be specifically made for digital cameras). Good luck fellow Pentax user!
EDIT: Just checked it out and B&H has the 16-45mm on for $309.95 after rebate.