As posted in this thread:
http://www.subsim.com/radioroom/showthread.php?t=133495
By running sub school (and not any saved games), I've definitely confirmed that it is a TM issue, in combination with whatever possible Vista related bugs and/or video driver issues I have on this machine. Rolling it back to stock I got the usual pretty partly-cloudy blue/white skies, reinstalling TM, I got the bleached-out look again. It "switched" to this halfway through my last patrol and has been persistent ever since. I'll try updating the video driver next and pray that works.