When you go download jQuery UI 1.8.2, you have the option to add the position plugin. If you haven't heard of it, it's new to jQuery UI and described as "A utility plugin for positioning elements relative to other elements". It's a nice helper for making elements like a modal window, tooltips and other positioned items.
Since it can't be downloaded as part of UI 1.7.3, My initial instinct was to assume that the position plugin needs jQuery 1.4+ in order to work. As a test I downloaded only the position plugin, and tried it out with jQuery 1.3.2. It seemed to work fine, and wasn't too shabby of an add-on since it's ~4k (minified, not gzipped).
I work on a lot of different sites, and don't always have the latest version of jQuery available, so it's nice to know that I can use this with older versions of jQuery. Also a plus is that I can grab the UI Position plugin as a standalone item without the bulk of the UI core.
Here's a proof-of-concept I whipped up using jQuery 1.3.2 and the UI plugin. The only hiccup was that the first time I called the plugin it didn't center the div correctly in Chrome... But if I called it twice, it was fine. Weird and kludgey, but hey it works... even in IE6. Since this is a useful plugin, and apparently not tied to UI, I'd love to see it included in jQuery Core.