The willingness of US investors to back important and disruptive new business ideas has been the envy of the world. From the providers of the earliest stages of "seed" capital to investors on Wall Street, the country's financial system has long underpinned the innovation that has been central to US economic success. But even as a new internet boom takes hold in financial markets, important changes are underway.

Other countries have learned the lessons of US venture capital, and stock markets elsewhere have at times proved a more reliable source of cash for ambitious young companies. Changes in the needs of promising new businesses are also contributing to an upheaval in the way start-ups are funded. From the emergence of "super-angels" to the rise of secondary markets for private company stock, the financing system is itself going through a period of disruption, with uncertain consequences.

Above all, the risks of a new boom and bust in the tech world have risen sharply as investors have been drawn by the hopes of big profits from the latest wave of social networking, mobile and "cloud" computing companies, despite growing economic uncertainty.

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