The "Iran Debate" is no longer just a debate about Iran. It is now the "Iran-Israel Debate", as the two are now inextricably linked. Due to Netanyahu's chest thumping, the Obama administration must now address the problem of Iran while hushing and simultaneously supporting a belligerent Israel. That's not to say that Obama is without options.
Take that, Bibi! Though the relationship between the US and Israel is strong, it might not be as strong as that "special relationship" between the US and the UK. For starters, we've fought two world wars alongside the UK. But most importantly, the US and the UK don't always agree on the issues, but we respect our differences of opinion. The US cannot, on the other hand, publicly disagree with Israel without significant political backlash (there is no American-British Public Affairs Committee). It's like that couple who can bicker and move on, versus that other couple who disagree on major issues but let their true feelings fester for years on end (and probably don't have much sex [don't read into that too much]).
Having David Cameron come out and support the US position is a good tactical move. It demonstrates distance between the US and the Israeli position, without actually saying it. Furthermore, Cameron is allowed to say things that Obama could never get away with. For example, "We've been very clear; If there was an Israeli strike, we wouldn't support them."
Score one for reading between the lines.