http://www.techcrunch.com/2010/01/31/google-labs-search-gmail/

<<Orli Yakuel noticed that Google has quietly added a new icon in the ‘Compose Mail’ window of its free webmail service Gmail, enabling users to run search queries from within the interface and insert results and URLs straight into drafted e-mails or open chat conversations.>>

http://cache0.techcrunch.com/wp-content/uploads/2010/01/gmail-icon1.png

Official blog post:

<< ...Finally, as some folks have already noticed, I added a search button to the toolbar when you're composing a message.

If you've got text selected when you click it, Gmail will search for that text. If you don't have text selected, you'll just get a search box where you can start typing. >>

http://gmailblog.blogspot.com/2010/02/improved-google-search-in-labs.html