So Jonas has gotten this working for WordPress, but I have some ideas of how to use it, somewhat differently, with Movable Type. I couldn't code Perl (or anything else really) to save my life so here is just the idea.
In MT admin > entry screen.
a) above the keywords field, add a checkbox to enable "Yahoo! Terms keyword generation"
(We don't wanna start creating new columns/rows/tables in the MT database so let's use Keywords, cause, well, that's what they are.)
b) when an entry is posted, after MT has done everything else it already does, and before we get the "operation completed" message, submit the title, entry and extended entry, as well as the generated permalink (why not... this will notify Yahoo! of a new entry, give it the URL and the text so it doesn't have to come and crawl for it...), get the XML reply of generated "Terms" and save them in the keywords field.
From here, there's all kindsa stuff you can do. Use the keywords for (yawn) header meta-keywords tags, Technorati entry tags or, with a bit of template wizardry (and either some yet to exists plugin, or some CRON and PHP code), generate a weighted list, which you can display as an overall "zeitgeist" (I prefer "what my blog is thinking") affair, as I made for Joi's sidebar. (I generate Joi's global keywords list using the MTKeywords plugin to create and array which I then parse with some PHP.)
Ken McClosky released MTYahooSearch which talks to Yahoo!'s API, but it seems to only pull stuff into templates. Cool, but I want to capture and save the Terms results along with the entry. Why? Because it's relevant?
Of course, as someone very smart recently said to me:
"With natural language processing, you end up twiddling a lot of knobs to get what you want."
... so whether you trust Yahoo! to generate relevant keywords is a matter of personal faith.
(Two days in a row staying up till dawn, and I still have an entry on folksonomies in Drupal keeping me awake.)