Compare with: http://en.wikipedia.org/wiki/Display_postscript pioneered for NeXT; a derivative is used by MacOS X.
NeXT did more with it, and clearly NeWS failed in the face of X windows in Sun's market.
Could it be someone suggested "NeXT" as a prank?
I manage to take screenshots as pdf:
defaults write com.apple.screencapture type pdf
killall SystemUIServer
Apple pushed (weakly) a "Resolution Independent" interface in 10.4 (I think) that would have promised drawing the screen at whatever DPI you want without looking terrible (and giving people who want larger text a better option than decreasing resolution)
It didn't really happen; though, and recently I remember seeing them push pixel-doubled assets, ostensibly in preparation for desktop retina displays in the future.
I didn't know GTK3 was vector drawn - that's really awesome!
For examples, compare the sidebar Finder icons with their full size counterparts, especially in the Places subsection. Some are scaled down versions of the larger icons (Documents, for example), whereas some are stylistic extractions (Downloads becomes a green download arrow in a circle).
I'd hope that when 2x mode comes to OS X for high resolution displays, when you take a screenshot in 1x mode it would use the 2x mode assets to create a higher resolution screencap, but that does sound like a lot of work for a very minor feature that would increase screenshot size.