I think the financial downturn is part of the problem, but I am sure it is a handy excuse anyway for manufacturers who have underperformed within F1 to pull out.
Honda pulled out at the height of the financial crisis, but were seriously underachieving in F1 and I think to save face pulled the plug bases on the economic crisis, despite the guys probably pleading that they had a great car for 2009 (Despite what they say, Honda must be kicking themselves). Honda are a team whose sales I really cannot see being affected by F1 or not, as it is not like they make sports cars who people might judge compared to the performance of their F1 team.
BMW probably did not realise the problems involved in running your own F1 team. Yes, they invested heavily into Hinwil and really modernised the Sauber team, but again, relatively, they underachieved and I think most would say it was criminal how they just stopped developing a car midway through the 2008 season when they really had a strong chance to win the title. Yet despite that, their 2009 car was horrible (but it still came 6th overall as a team). I think the bosses in Munich just thought F1 was not showing them in a good light. But I think they probably would be in a position to use more F1 related technology in their cars than the two Japanese manufacturers.
So, Toyota... They blamed it on their first financial loss in 7 decades of operating. Never mind no win in 8 years! I think their claim for leaving due to the downturn is possibly the most spurious given they are one of (Not sure if still) the biggest selling car company worldwide. Furthermore, Japan I think I read is out of recession now, so surely their finances would go back into the black for the next financial year? This was purely a face saving action, it must be humiliating having spent what, close to $2b and not having won a race?