I have read many new things about css3, although it has been around for quite a while, and a lot of "new" things aren't really all that "new"
I believe the selector you are refering to is "background-size"
it was a working draft back in 2002
http://www.w3.org/TR/2002/WD-css3-background-20020802/
so its more the fact it took you 9 years to find it, then "it" taking so long to be available. of course backgrounds and borders are now in the candidate recommendation phase, see
http://www.w3.org/TR/css3-background/ for more details.
it is available, and works in the latest chrome, safari, opera and konqueror, I believe it is also working in FF4.