Apparently, when you “Stop all crawls” in the Shared Services Administration: Default Web Site > Search Settings > Content Sources of SharePoint Central Administration, the next crawl will be a Full Crawl. The warning told me! Wait, I mean the warning that should have been there.
On the Manage Content Sources page, point to the content source you want to crawl, click the arrow that appears, and then click Stop Crawl on the menu that appears.
A message appears warning you that stopping this crawl will result in the next crawl of this content source being a full crawl.
So that's why you see a full crawl when it should've been incremental. Or maybe I'm the only one who doesn't get this error?