Bumping this because I found this:

http://www.gamespot.com/news/6214693.html?om_act=convert&om_clk=newstop&tag=newstop;title;5

SONY COMPUTER ENTERTAINMENT SALES (through June 2009)
PS3 hardware LTD - 23.8 million
PSP hardware LTD - 55.9 million
PS3 hardware FY09 forecast - 13 million
PSP software FY09 forecast - 15 million
PS3 hardware LTD sales through March 31, 2010 (predicted) - 36.8 million
PSP hardware LTD sales through March 31, 2010 (predicted) - 70.9 million

If the prediction comes true and they have 71 million by March 31, 2010, then it's only another 10.5 million until it passes the GBA

That PSP figure is wrong - Game Spot combined PSP production shipments with shipments to retailers, which is redundant.

The 50m PSPs shipped to retailers that you cited is through January - PSP was at 49.7m through December 2008, and shipments in the past two quarters bring it to 53m or so. Sell in is what Sony calls sales to retailers, i.e. shipments.

You can see how big the differences in shipments vs production shipments can be by comparing production shipments http://www.scei.co.jp/corporate/data/bizdatapsp_e.html for the March 2007 year on there (25.4-17.0m) with the sales to retailers (shipments) reported here for the same year http://www.scei.co.jp/corporate/data/bizdatapsp_sale_e.html.

Fair enough, but even if you're right and PSP is at 53 million, it's still plausible that it can catch up and possibly pass the GBA.

Yeah if the Go doesn't fail. I think it can surpass the GBA.