Initial public offerings of venture-backed companies raised $1.2 billion during the first quarter, double the amount raised during last year’s first quarter.

The data, compiled by by VentureOne, was confirmed in a second report by Thomson Financial and the National Venture Capital Association. That report showed venture-backed companies raised $2.09 billion through IPOs — but that included the IPO of Clearwire, which was backed mainly by corporate investors such as Intel and Motorola.

However, acquistion activity was down.

Acquisitions of venture-backed companies numbered 62 during the quarter, compared to 104 in the first quarter of 2006, the Thomson/NVCA study found. However, the average disclosed deal size for the first quarter of 2007 was $161.2 million, one of the highest quarters in the last five years, it found.

Here is more data on the Thomson/NVCA study.

Here is a helpful spreadsheet of the DowJones data (downloads Excel file.

“This quarter may mark a subtle shift in the exit mix for venture capital,” said Mark Heesen, president of the NVCA said in a statement. “There appears to be a crack in the IPO window which changes the psychology of the market.”