It's been one of the most requested features, and I've seen plenty of comments here at TUAW bemoaning the absence of this capability. The newly added feature allows searching through all projects while still being able to confine the query to various scopes, including Title, Notes or Tags. This, of course, makes the tagging system in Things even more useful, allowing all tasks with a given tag or tag combination to be listed. Personally, I'd say this is the most important new feature added to (the desktop version of) Things since iCal sync. I'm betting we'll see more of these highly-requested features implemented as the development pace amps up to meet the Macworld deadline.
A blog post at Cultured Code details all of the new features. You can download a free trial at the Cultured Code website, and a license can be had for $39USD ... if you sign up for the newsletter before the January 6th launch (after which the price will be $49USD).