By "portable gaming system", I mean either a dedicated gaming console, or a cell phone. The interesting thing about the cell phone option is that while dedicated gaming systems come out rarely, cell phones are increasingly on the PC-style plan of coming out as rapidly as possible with as much hardware stuffed in them as possible. So while guessing when the dedicated portable gaming systems will pass the XBox 360 requires reading minds, guessing when cell phones will actually pass the XBox 360 is not so hard.
Using this logic, I think we can expect the PS4 and XBox 720 by 2013 at the latest, not because the cell phones will be quite there by then, but because the cell phones on the horizon in late 2012 to 2013 will be able to exceed them. And especially for the PS and Xbox line, that's an embarrassment I don't think they can put up with and still maintain their image, no matter how unhelpful the next generation of graphics will be to their market or how expensive it makes the games. Unless they just throw in the towel, which, honestly, isn't necessarily a bad bet for the PS5 or Xbox 1080, but I think we're going to have one more conventional generation of consoles whether or not it's a good idea, after which we may discover they don't have a niche anymore.
PS1 - 1994
PS2 - 2000
PS3 - 2006
The average works out roughly the same for Nintendo, and since Microsoft only has two data points it's not really worth including them.
PS: The XBox was discontinued early due to licensing issues. Microsoft was unable to drop the production costs as far as they would like so they decided to release the XBox 360 early.