The IRS says the one-billionth tax return in the history of e-filing was e-filed recently.  From an IRS news release:

IRS e-file has reached a major milestone as it passed the one billion mark for individual tax returns processed safely and securely since 1986.
The Internal Revenue Service’s electronic filing program started as a pilot project in 1986 and became available nationally in 1990. Prior to the April 18 deadline, IRS e-file passed another high point as more than 100 million individual tax returns were e-filed during the 2011 filing season.
“IRS e-file is a good deal for taxpayers,” said IRS Commissioner Doug Shulman. “The one billion milestone means e-file has delivered real services to taxpayers, including faster refunds and more accurate tax returns. And because an e-file return costs us 20 times less to process than a paper return, this program means a more efficient government that has saved America’s taxpayers hundreds of millions of dollars.”