It's hardly surprising that Toyota have gone - The company that canned the Celica and MR2 as part of its drive to have very low CO2 emissions across the range. That never quite sit right with formula 1, and toyota's position with hybrids doesn't really need motorsport support for sales. Basically, the same position honda were in.
As for GM and opel/vauxhall... I'm surprised it has taken them this long. The opel developed cars are rapidly spreading in the US (eg Saturn L300 was the Vectra estate, Saturn Aura and Chevrolet Malibu are both Epsilon platform based, Saturn Astra is exactly that, etc) - They offer a more appropriate fuel economy and practicality for the current economic situation than indigenous models (eg W-body models such as the Chevy Impala and Buick Lacrosse). Many new models are based on opel platforms (eg the Chevy Cruze is based on the Delta II platform, which is the new astra).