A recent poll by the research firm Harris Interactive  indicates that the number of bills paid online is rapidly approaching the number paid by check:

Other = cash, debit cards, or other payment methods.
Source: Harris Interactive data (quoted in The New York Times)

Keep in mind that CheckFree (a leading providor of online bill presentment and payment services) commissioned the study. Although the findings correlate with other recent studies.

A number of factors drive the increase in online payment:

• Increaing numbers of consumers online
• Decreasing fees charged by CheckFree and Metavante, reducing bank costs
• Recognition by banks that online bill pay customers are more proftiable than other cutsomers because they are more likely to utilize other products
• Increasing bank willingness to absorb cost, leading to decreasing online bill pay fees for consumers (free at some bank sites)

The rate of adoption may decrease, however, as new internet users (late adopters) may take 2-3 years to grow comfortable online and attempt online bill payment.

Pay Bills With a Click? More Americans Are Doing It and Banks Are Loving It
E-Commerce Report
By BOB TEDESCHI
The New York Times
May 29, 2006