UPDATE: Here’s the official word from Yahoo spokesperson Terrell Karlsten. They’re claiming they don’t upgrade users to IE7 and their reasons for switching your settings? Everyone else does it.
Yahoo! Messenger’s update process does not download IE7 to a user’s computer. As part of the Yahoo! Messenger update process, people have the choice to download Yahoo! Toolbar, set Yahoo.com as their homepage and set Yahoo! Search as their search engine. This is an industry-wide practice….IE7 is not bundled into the Yahoo! Messenger update process.
UPDATE 2: Jarrod’s added a comment that suggests that IE7 was not added as part of the download, as he first described to us. All other aspects described below are known to be accurate, including changing the default browser to IE, changing the default homepage to Yahoo and changing the default search engine to Yahoo. The post has been updated.