With the regional sales numbers for last week which we just got for EU/others and will get for NA in a few hours or so, we'll see that the Xbox 360 has passed 19 million consoles sold world wide.
Despite only 2.5 years on the market, the X360 already has an installed base less than 5 million short of the original Xbox. The 360 has sold around 25% faster than it's predecessor since launch, but has an even stronger momentum - comparing Jan-May 2004 for Xbox with Jan-May 2008 for X360, the X360 is actually selling 73% more units per week!
EDIT (29 May): after the data adjustments made in the last days on VGC for the X360, the number is only 65%. Bear in mind though that the year of comparison, 2004, was the Xbox' best selling year with 6.8 million units sold.
This is truly a remarkable achievement, especially considering that the X360 today costs $349 in North America, compared to Xbox which cost only $149 at the same point in time.
The X360 is indeed alive and well.