here is another option that folks seem to like plus it's not expensive at all:
Pentax Lens Review and Specification Database - 100-300mm F4.7-5.8
I looked on eBay and it can be found for $99-$129...certainly not the best of the best but sure seems to take nice shots if you search here and Flickr...
It will not be the fastest AF lens around nor will it be good for night or low light, but for shooting in decent daylight or even well lighted places using a monopod or tripod I bet it will do a fine job.