Just the feature I was looking for. Summer 13 introduces a feature whereby you can search for keywords in chatter posts. Navigate to the Chatter tab. Select the chatter section on the left side bar to show all feeds. Click on the magnifying glass just above the feeds and enter the search keyword. All the chatter posts containing the keyword will be displayed with the keyword highlighted in yellow.

Search for a keyword in chatter post
Search in Chatter Posts