Perhaps the most important caveat about Prop 19 is that it only legalizes marijuana under state law.
The federal government's prohibition will remain in place, so the federal government could still enforce that prohibition in California. This happened for medical marijuana under the Bush administration, and under the alcohol Prohibition of the 1920s and early '30s, when the federal government enforced prohibition in states that had not banned alcohol.
Prop 19 advocates have assumed that the Obama administration would tolerate legalized marijuana, as it does now for medical marijuana. This always seemed unlikely, however. Federal abdication would give the Republicans a huge issue and suggest that states can ignore federal laws they oppose, such as "Obamacare."
via cnn.com
Actually, States can ignore Federal laws that are not constitutional. The process is called Nullification, and it's codified in the 10th Amendment. This applies to everything, including marijuana legislation and Obamacare.