In order to begin work on a PhD, you must have already earned a Master's.
That means you should already be thoroughly knowledgeable about the field as it exists today.
That's what Masters means -- You have "mastered" the field
Hi pullbangdead,This isn't always true. In my department, >90% of people enter straight into the PhD program with only a bachelor's degree, and in fact never earn a masters degree. The goal is a PhD from the beginning. The department won't fund anyone who wants only a masters, and the same is true in many other places. Departments don't want to invest thousands of dollars in you only for you to leave after a year or 2, so you commit to the PhD program going in. Once you're committed to the PhD, why bother with the masters?
That's the case for me. I started my PhD work with only a BS degree in summer of 2008, and I am currently on track to have my PhD in ~14 months. I don't have a masters degree now, won't have one before I finish my PhD, and will likely never have one. If I ever do, it'll be in a different field...maybe an MBA one day, who knows?
I agree about asking your adviser first, though. Generally a good place to start.