There is now a rattlesnake vaccine on the market from Red Rock Biologics called Crotalus Atrox Toxoid.  It is for prophylactic use to reduce signs of illness and death rate caused by rattlesnake bites.  It has been in use in California since 2003 and has had an excellent safety record.

The initial vaccination is two doses spaced 4 (3 to 6) weeks apart.  Annual boosters are recommended around 1 month prior to potential exposure.  Boosters can be done as often as 4-6 month intervals in high-risk situations.  The vaccine may be used in puppies 4 months of age, as well as pregnant and lactating bitches.

Envenomation from a rattlesnake bite leads to: swelling, bruising, pain, weakness, nervousness and excitability, vomiting, breathing difficulty, bleeding, weak pulse, heart failure, secondary gangrenous infection, paralysis, unconsciousness and possibly death.  Size does matter; the smaller the victim, the more serious the consequences.  Treatment (antivenin) within 4 hours can reverse many of the systemic effects of a rattlesnake bite, but has minimal benefit on local tissue damage.  A vaccinated dog will already have antibodies, which will decrease the onset and severity of signs, both systemically and locally.  This protection can be overwhelmed with a highly venomous bite though, so even vaccinated dogs should be seen by a veterinarian.
The composition of rattlesnake venom is very complex and varies between species, individuals, and even within an individual over time.  The vaccine is derived specifically for the Western diamondback (Crotalus atrox), but research indicates there is cross protection with other US species.  Initial studies have shown excellent protection against red diamond, western, and western diamondback; good protection from pigmy, timber, copperhead, and massasauga; fair for the eastern diamondback; and likely protection from the sidewinder.

For more information on the product, go to: http://www.redrockbiologics.com