Supply and demand, baby! These fluctuations happen; with a major new advance in CPUs just released (C2D), a lot of people need to buy new RAM as they upgrade from DDR to DDR2. Vista, coming soon, will finally be able to use more than the 2-3GB limit of XP, so people are buying more RAM with new systems they are buying for Vista, and so forth.
One thing that i learnt in A level Economics is that as the price increases, the supply increases. So eventually we will have saturated supply so the prices must fall at one point. Or is that just my hopeful thinking?