And in answer to people's concerns, one thing which Northern Rock, Bear Stearns and Lehmans all had in common was that they had no (or a very small) deposit base. What that effectively means is they did not have a large stack of money which people had given them to look after in the way that RBS, HBOS, Barclays, HSBC, JMorgan Chase, Citibank etc. all have as a result of their retail banking operations. They were solely investment banks. RBS, for example, has just under £600 billion of its own capital in the form of retail deposit accounts.
What brought down the banks above was not the losses on so-called 'toxic' assets. The majority of these are only paper losses, i.e. theoretical losses, not losses which they have to pay for. What absolutely did bring them down was that other banks stopped lending to them which created a vicious circle leading to insolvency.
i.e. Banks start getting concerned that Lehman might get into financial difficulties -> people reduce lending to Lehman -> Lehman finds it harder to settle the obligations which it does have -> banks get even more concerned that Lehman might get into financial difficulties.
Ultimately, with banks refusing to lend money to these institutions they had no money to settle the outstanding obligations which they did have, and they were forced into administration. In the case of the banks with big retail operations, however, they have their own large pool of liquidity to draw upon so they're not exposed in the same way which Lehman and co were.
Plus, and equally importantly, because the US institutions did not have a retail deposit base the direct fall out from the failure of these banks is the institution itself and other financial institutions not the man in the street. If they did, it's very likely that the government will step in to protect the consumer, as they did with Northern Rock - be it directly through nationalising or indirectly by requiring that other institutions support it/ providing central funding etc. I really wouldn't be worrying about the guys like HSBC and RBS at the moment.