While I'm all for new technology, I do agree that it is way to early for another DX revision. Out of the small handful of DX10 games, only a select few can be maxed out on anything but the highest end video cards.
The time frame is about right. DirectX10 has actually been available for two years already since Vista was launched in late 2006. It feels like it only came out recently because it took a long time before any significant number of gamers upgraded to Vista and for any games to take advantage of the DX10 features... but DX10 will have been available for 2.5 years by the time DX11 is launched by mid-2009.
While DX9.0/SM 2.0 has been available since late 2002, SM 3.0 capability was only added with DX 9.0C in mid-2004, meaning it was also out for about 2.5 years before being superseded by SM 4.0/DX10.