Rasmus already posted on his blog (with a great summary), so I won't bother rehashing everything, but I'm going to add a few things to his description as this is a product feature I've been closely involved with. Go read his description, then come on back... Done? okay...
Kinda cool, eh? One of the nice things about this (and I really need to emphasize this fact) is that it encourages site owners to provide more machine readable data on their sites via microformats and other feeds. In the short term this is good for Yahoo! since we're already setup to consume and use this data, but in the long term it's good for everyone. Other search engines get more data to give you better results, but more importantly all walk of app developers get richer, fuller access to meaningful data without having to write awkward page scrapers. The web just got a little more open.
Okay, I'll put the pom-poms down...
As Rasmus mentioned, you John Q. Developer, get to decide what data gets used and how it gets used. With the addition of all this new semantic data you can just walk through Yahoo's massive index (getting bigger thanks to microformats), but you can call out to external data sources too (yes, there are measures in place to limit baddies from DOSing innocents with Y! bandwidth).
Once you've got data, you get to write real code using a proper turing complete language, not yet-another-macro-language-that-lacks-proper-control-structures. Of course, it's not ALL of stock PHP. Again, we're not going to let you use Y! servers to hurt innocent users. Stop thinking bad thoughts... Okay thinking bad thoughts is fine, just don't act on 'em.
The best part is, it's my team that's been working on (part of) this. We can't take all the credit as there have been a lot of moving pieces, and I'll gladly share the blame with someone else (that's what product managers are for). I'm certain that bug you just found on the alpha version was introduced while I was on vacation. No no... it must have been the other Sara(h) working on that piece. WHY ARE YOU CHECKING THE CVS COMMIT LOG??? SOMEONE STOLE MY PASSWORD! I SWEAR! 
Excited yet? Then you're a bit of a geek, go find a (boy|girl)friend... Can't find a snuggle-buddy? Live in the SF Bay Area? Come to the developer event and launch party! (it's not just a soiré, it's an event) I'll be there, and I'm worth it all on my own! Right? right...? Ooo?
