There are actually quite a few dog-friendly campsites at Northern California beaches. Most state parks allow dogs in the campgrounds, but they're usually not allowed on beaches or trails. There are also county parks and private campgrounds that are dog-friendly.
Since several people have asked about this topic, I did a bit of research and compiled a list of dog friendly beaches and campgrounds in Northern California
. On that page, you'll also find information on the pet regulations, and links to the dog policies of the various agencies that operate Northern California beaches and parks.
Humboldt County is clearly the most dog friendly. Not only do they have several campgrounds where dogs are welcome, but dogs may also go unleashed on many Humboldt County beaches.