If you need to search for something you chatted about in Skype, but do not remember exactly who or where you chatted with, it's hard to do right now. But here's one way to do it with Spotlight.
The latest version of Skype for Mac has a history button for each contact -- click the Gear icon, then select View Chat History. When you select this menu item, Skype creates an HTML file in your user's library; you'll find it here: ~/Library » Application Support » Skype » skype_acct » IMHistory.
Once the file's been created, Spotlight will index its contents (you'll have to include System Files when searching via Spotlight in the Finder), thus making chat conversation history searchable for any contacts for which you have generated a history file.
[robg adds: Based on my testing, it seems this history file isn't updated when new conversations occur, which means it will be out of date relatively quickly. Still, this at least provides one way to search Skype chats.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091113070817781