On CrazyBoards, the support and education boards I help moderate, people with bipolar disorder who rapid cycle are pretty common. Rapid cycling refers to having four or more distinct mood episodes a year, although usually what people are referring to on CB is more frequent. This may reflect who winds up on support boards or may reflect actual distribution, I don’t know right now.
It looks like almost always, rather than being a type of bipolar or a set pattern that’s persistent throughout the lifetime, rapid cycling is transient. It may or may not be a result of kindling (untreated mood episodes getting worse over time). We’ve known this for a long time, even if it isn’t widely known among patients: the studies referenced here (Goodwin and Jamison again) mostly range from late 1908s to mid-1990s.
Some additional things I’m wondering about:
If rapid cycling isn’t a type of bipolar disorder, is degree of variation in cycle lengths? For example, some people have a classic pattern consisting of a mania in the spring and a depression in the fall, and don’t deviate. But other people might sometimes have years between episodes and sometimes days between episodes.
If rapid-cycling is a result of kindling, and rapid-cycling is transient, does that mean people are getting kindled and later unkindled? Spontaneously, or due to medication, or both?
empirical insanity 
My doctor said I was a rapid cycler type 1 because I had allergic reactions to ALL of my medication since I was 11. I’m nineteen now. He said it fried my seritonin gland, and in milder bipolars the gland is of course, but mine was erratic.
That’s pretty interesting! I’m guessing he means your pineal gland. I’ve never heard of allergic reactions damaging the pineal gland, and I admit I’m a little skeptical, though.