I try to look at things "broadly", as when you look to closely you will see seasonal variations and increases from game launches which can skew trends. Looking at it yearly seems to eliminate that.
With the Wii, if NSMB Wii launched in July, I would guess that this Christmas would have been worse than last years, but late summer and early fall wouldn't have been as big a decline. Overall, it looks like the Wii has peaked, in my opinion anyway. If it hasn't, and this summer was just a lull, then the predictions others made earlier of lifetime sales of 150 - 180 million are still possible, and NSMB Wii could go on to sell 30+ million to become the highest selling non-bundled console game.
Also, big game releases after a peak seem to just slow down the drop off (for example, FF XII and GoW2 did not cause another peak with the PS2). So I'm not expecting huge hardware boosts from SMG2, Metroid or Zelda this year.
The Playstation brand is a bit of an oddity with the slim launches, as they seem to rejuvenate its sales. I sometimes wonder if people held off buying the PS3 until the slim launched, depressing earlier sales, or if it was all due to its price finally becoming reasonable for most people. But I digress, and I don't want to completely hijack this thread.