I agree Obama's reaction is politically motivated. It seems that some American's have unrealistic expectations of their not-quite-so-new leader as they call on him to "do something". Instead of being honest and pointing out that there is little he can do and this is the kind of risk you run drilling off-shore, he's decided to casting BP as an evil foreign owned aggressor who is carelessly polluting the hither-to unsullied US waters with its wicked British gloopy oil.
Many people have correctly pointed out that BP is owned in almost equal measure by US and UK shareholders. So, as many Americans will suffer from BPs falling share price as British. They can all thank Obama for making matter worse on that front. But even if you accept BP is a British company, the US doesn't exactly have a clean record on marine oil spills itself. Indeed, you'll see from the previous link that a US company has failed to completely quell much smaller leaks caused by the destruction of an oil rig back on 2004!
However, I do wonder how wise it would be to lower our own diplomatic language to the same desperate levels as the US President's. In fact, I question the actual degree to which Obama is being anti-British (on this occasion). A lot of the "anti-Britishness" has been exaggerated and conflated with other seemingly anti-British actions, like sending the Oval office's bust of Churchill back to the UK and backing Argentina's Falkland Island claim, to make a more interesting newspaper story that neatly fits an on-going narrative about the death of the UK/US special relationship.
I think there's some truth to this narrative; Obama is the least well predisposed President for a long time towards the "special relationship". However, I doubt Cameron wading in and picking a fight with America over an issue neither government has any real influence, will help reduce the de-specialisation of the UK/US relationship. In fact, all it will do is make things worse for BP. It would validate, in the minds of undecided Americans, any intention Obama had to pin this on the Britishness of BP and harden attitudes against them. BP's commercial interests in the US (if not already fatally damaged) would be damaged further.
The best Cameron can do is privately ask Obama to tone down the rhetoric and be more constructive regarding his criticism of BP as an organisation. Standing up for Britain, in this instance, would be best done using quiet diplomacy. If Obama ignores it, then so be it. BP will have to get on and deal with it. Their commercial interests are the only interests that should be at stake here. Let's not drag UK/US international relations into this any more than Obama has done so already.
UPDATE 18:00: Downing Street Statement on Cameron/Obama Telephone Conversation
"The Prime Minister and President spoke for over 30 minutes today."
"They discussed Afghanistan, where the Prime Minister briefed the President on his visit. On Iran, they agreed on the need for the European Council to signal tough measures in support of this week's clear message to the Iranian regime from New York that it must halt its military nuclear programme."
"They also discussed preparations for the G20, where the US and UK are also working together closely."
"The Prime Minister expressed his sadness at the ongoing human and environmental catastrophe in Louisiana. The President and Prime Minister agreed that BP should continue - as they have pledged - to work intensively to ensure that all sensible and reasonable steps are taken as rapidly as practicable to deal with the consequences of this catastrophe."
"President Obama said to the Prime Minister that his unequivocal view was that BP was a multinational global company and that frustrations about the oil spill had nothing to do with national identity.The Prime Minister stressed the economic importance of BP to the UK, US and other countries. The President made clear that he had no interest in undermining BP's value."
"The President and Prime Minister reaffirmed their confidence in the unique strength of the US-UK relationship."