Hi Steve-- thanks for your comments, it’s always great to have the source of the article stop by! Yslow is a fantastic tool, and it reflects very well on Yahoo! to release something so helpful to the community. Anything that gets this many people talking about ways to improve web performance is a net public good.
However, as noted by Matt’s comment directly above yours, caching is something that you have to be very careful with. I accidentally turned on the Expires/Cache-Control header for ALL my content for about an hour (whoops!) before I realized what I had done. Thus, everyone who was unfortunate enough to visit in that window of time won’t see any changes on the homepage until the cache expires, 7 days from now.
Totally my fault, of course, but I do think this is exactly why IIS defaults to never setting an Expires/Cache-Control field on any content.