ClickZ : Top Reasons for Abandoned Online Purchases
Apparently online retailers are losing approximately $3.8 billion in revenue due to the fact that four out of five consumers give up before completing an online purchase.

The reasons given for abandoning purchases are varied. Of consumers who bailed out of a purchase, 52 percent said that too much data was required.

http://clickz.com/cgi-bin/gt/article.html?article=3178