I had an idea. I have a feeling that officially 3.1 will be out in mid-September with the iPod touch 3rd Gen. Here's why.
A history of iPhone OS releases:
1.0 June 29, 2007
1.1 September 14, 2007
1.1.1 September 27, 2007
1.1.2 November 12, 2007
1.1.3 January 15, 2008
2.0 July 11, 2008
2.1 September 12, 2008
2.2 November 21, 2008
2.2.1 January 27, 2009
So as history suggests we'll see updates like this:
3.0 June 17, 2009
3.0.1 July 31, 2009
3.1 September 15, 2009
3.1.1 November 24, 2009
3.2 January 26, 2010
4.0 June 8, 2010
Of course... this is all speculation but it could give you an idea of Apple's usual release schedule.