I haven't really covered this series as the sum total of the information comes from a poster (reproduced at right, click for larger size), but it seems there is a mountain bike race at Lake Sonoma on May 17th. And no, it doesn't appear to be one of the Lake Sonoma Series (like this weekend's race).
The poster reads "May 17: Lake Sonoma, Mountain, 25 miles or so" and it starts at 10 AM behind the Union Hotel.
Anyway, this is the last race in the 2008 Grasshopper Series and (scant) information can be found over on the SoulCraft blog or the Grasshopper Adventure Series blog.
Thanks to reader Daniele for the tip.
UPDATE: just got an email update on this: "oh, keeper of mountain bike race info: a fair bit of concern on the posting of the last grasshopper on your blog... it's not a race, just an organized ride, and we'd hate to see folks making the long haul up there expecting something different. no support, no course markings, no nothing."