It's not a pyramid scheme in the exact sense, but Bitcoin's more valuable with a wide subscriber base, and early adopters have a large advantage in terms of acquiring portions of the Bitcoin space cheaply. The changes in the investment-to-returns ratio over time that these pressures produce do end up looking a lot like a pyramid scheme's in the case where Bitcoin fails as a currency, even though you can make that investment in GPU cycles rather than dollars.

If it succeeds as a currency, of course, that curve ends up looking more like what you'd see in a gold rush or another emerging market. So some of this looks like dueling prognostications to me, or even just different levels of cynicism regarding the concept.

(By way of disclaimer, I don't currently own any Bitcoins.)