So I am planning on heading up to Napa this weekend for the Howell Mountain Challenge (since I missed the Napa Valley Dirt Classic) and I thought I would post as much scouting information as I could find. Now, most of this information is based on last year's race, but I have on good authority that the course is substantially the same this year.
First up is the GPS data from last year's race. While the overall data isn't the greatest (probably due to the tree coverage), it does show that it's about 1500' of climbing and 10 miles per lap
The complete GPS data is available over at Motionbased.
Other data points are race reports. I think the best ones are Team WrongWay's 2005 coverage. Multiple people talk about their race in great detail.
Finally, for those visual learners, check out the photos from last year.