I assume that this list only encompases US releases, correct?

There's a correlation between strong library increases, and Japanese support, imo.

For the 360, there was no J-support early on, aside from DOA4, EM and RR. This rather hampered the 360's drought. However, I can't remember - didn't DOA4 and Full Auto release before you note the start of 360 games?

Correct, this graph only encompasses US releases. I didn't want to double- or triple-count international releases of the same title, so I took this NA-centric approach for now. I may go back and try to do separate charts for the three major territories at some point. There might be some mis-timing of early releases, I plan to go back through the VGChartz history to see which week titles first appeared in the sales data and clean some of that up.

I'm also intentionally NOT taking announced/upcoming titles into account in any way. What actually counts, IMO, is titles brought to the store shelves; if a platform is failing or market tastes shift, or a completed title isn't up to critical snuff, sometimes the investment for retail distribution proves not to be worth it. (There were over a hundred titles announced for the Atari Jaguar that never made it to market.)

I do plan to update and improve this, just wanted to take a first crack at it while I had some free time over the weekend.

Updated with this week's data:

Again, this chart only represents North American releases.  I am trying to count all SKUs equally, including ports and remakes ("special" editions released at the same time as the regular edition are not being counted, but later "Game of the Year" editions and so forth are.)  I'm not paying any attention to downloadable titles.  My intent is to track how much retail shelf space and publisher resources are being devoted to each platform over time, in the North American market.

Trendwise, 360 still appears to be leading the pack in both library size and growth rate.  Wii picking up a little bit thanks to several releases last week, but still seeing "dry" weeks now and then.  PS3 still fairly linear, I expect the "flattening" currently showing up to work itself out in the next month or so.