I have gotten sick and tired of hearing people spew the absolute bullshit about how Monster Hunter 2G saved the PSP. It gave an incredible, and amazingly well sustained boost to the system, but did not cause the level of sales people attribute to it. The game like any other "system seller" caused a large spike followed by a relatively brief period of sustained sales before leveling off to the exact point it was at before. Don't take my word for it though, lets look at the charts.
http://vgchartz.com/hwcomps.php?cons1=DS®1=Japan&cons2=PSP®2=Japan&cons3=X360®3=------&start=39453&end=39628
The above chart is weekly PSP sales in Japan for this year. You might notice a fairly large spike right in the middle of it. That would be the release of the latest Monster Hunter game for the PSP. The intresting point for me is that the sales 2 months before, and 2 months after are roughly the same. The second bump prolonged the increased sales, and was likely due to another big PSP game being released. Perhaps it was the first Monster Hunter game that caused it.
http://vgchartz.com/hwcomps.php?cons1=DS®1=Japan&cons2=PSP®2=Japan&cons3=X360®3=------&start=39089&end=39628
Here is a fairly long term chart for weekly PSP sales in Japan. The first giant bump was the result of the first Monster Hunter game being released. Notice that it again that sales returned to the level they were at prior to the release of the game. You might notice that second, much larger spike in sales that did cause a sustained increase in PSP sales. Let's zoom in on that and see what is up with that.
http://vgchartz.com/hwcomps.php?cons1=DS®1=Japan&cons2=PSP®2=Japan&cons3=X360®3=------&start=39320&end=39628
Here we have a zoomed in graph at the above mentioned spike. September 15th saw one huge new release for a Sony portable console. That release was the PSP Slim. Notice how sustained sales go up from their previous low and to the recently sustained highs. It is selling at the almost the exact same rate after that release as it is today. It has been selling at that rate for all the inbetween months as well save for the normal boosts from big games and holidays.
Some of you may be wondering why I included the DS in that list. The reason is people seem to think market saturation of the DS has increased PSP sales. As you can see in the second graph though that is simply not the case. The PSP's boost in sales happened well before the decrease in DS sales due to saturation. The DS and PSP both had sustained amazing sales for 2 or so months before the DS fell off and the PSP kept chugging along.
The short version of this is the PSP Slim is what has caused the complete turn around in Japan. No one game has ever, or will ever cause a permanent increase in sales of a console. You need a wide selection of good games to sustain your sales, and have an attractive system at a reasonable price point.