Today at a taping of Late Show with David Letterman, the host announced he was retiring.  There's been no official announcement yet, but word of Letterman's retirement came through the Twitter feed of R.E.M. bassist Mike Mills, who is joining tonight’s musical guest Joseph Arthur.  Last October, Letterman signed an extension with CBS to stay on the air through 2015, so just because Letterman is announcing his retirement, it's not like he's hanging it up tomorrow.

As Vulture points out, David Letterman currently holds the record for longest working late-night host in TV history.  He spent 11 years at Late Night on NBC and 21 years at Late Show.  Even though he hasn't even officially announced he's leaving, feel free to speculate on his replacement.  Hit the jump to watch Letterman's announcement of his retirement.