Whatever you've been taking between 5/19/10 and 6/4/10- I want some 

Good improvement there!
Frankly, I've discovered the same thing. That is, when I ride the same loop with various challenges (climbs, sand, stairsteps, rockgardens, downhills) over the course of time, the familiarity of it encourages me to attempt things that made me nervous before and my time improves. But not as dramatically as yours. The loop I take I have used, off and on, for over 14 years and has become more eroded in spots and some areas are totally different due to flash flooding.
Sometimes familiarity does NOT breed contempt, it removes uncertainty and allows improvement.
Tzvia- rollin' slow...
Specialized Ruby Expert/mens Bontrager Inform RXL
Specialized SWorks Safire/mens Bontrager Inform RL
Giant Anthem-W XT-XTR/mens Bontrager Inform RXL
Fuji Newest 3 commuter/mens Bontrager Inform RL
Novara E.T.A commuter/mens Bontrager Inform RL