There is a lot of research in this area right now. The assumption that we should be bleeding every 28 days is being seriously questioned. In the past, women of childbearing age without access to Birth Control would spend a good proportion of their years pregnant or lactating, meaning no periods. Women might go for years with very few periods. This was not abnormal.
The argument goes like this: Mestruation is needed to flush out the matter produced by our bodies in preparation for conception. If we don't produce this stuff (like when we're on the pill), there is no need to "flush" it.
So now we are being given an option to have fewer periods.
Here is the article about the new pill:
Business Week article
Here is an article from WebMD on the concept:
Is Menstruation Really Natural (WebMD)