"The result is a slowing rate of adoption, with online banking households increasing by only 3.1 percent in the last quarter of 2005 -- the lowest increase in three years," according to Lisa Phillips, senior analyst at eMarketer and the report's author. "The number of online banking households as a percentage of total online households is pretty stagnant, too. It is expected to grow by just 4 percentage points between 2006 and 2010, from 45.4 million in 2006 to 56.2 million in 2010."

While that number may seem high, it is actually more conservative than the estimates of other researchers, Phillips said.

Web site security tops the list of consumers' concerns about online banking, she said.

"Being able to trust a banking site is extremely important to customers, with more than 87 percent saying in an Ipsos Insight survey that they wanted assurance that the bank would not sell their personal information, and 83 percent wanted assurance that such data was protected from hackers," according to the report.

"Security is not a luxury to online banking users, and it cannot be for online banks," Phillips said in a statement.