Search is one of the most requested features for Google Reader. Until this feature is implemented, there's a simple workaround: you can use Google Co-op to create a search engine restricted only to the sites you're subscribed to. Here's how to do this:
1. Go to this page that shows your list of subscriptions (in OPML format) and save it to your desktop.
2. Create a search engine at Google Co-op. Enter a name, a description, a bogus keyword (like "blogs"), a bogus site in the list of sites to search (like "www.example.com"). Click "Next" and then "Finish".
3. Now click on "control panel" next to the search engine you've just created. Select "Advanced" and upload the file downloaded from Google Reader in the annotations section.
4. (Optional) To access your search engine, you have more options:
* Integrate it into Google Reader using a Greasemonkey script
* Go to "Code" section and copy some code you can add to your site or add a gadget to Google Personalized Homepage.
* Go to "Preview" section and bookmark the homepage of your search engine.
* You can also go to the homepage of your custom search engine, right-click on the search box and add the search engine to your browser (for example: if you have Google Toolbar, right-click and select "Generate custom search"; if you use Opera, select "Create search").
You can use this to search the sites from any OPML file (most feed readers have an option to export the subscribed feeds to OPML). Of course, if you add or remove feeds, you need to reupload the OPML to Google Co-op to keep the search engine in sync.
Update (Sept. 2007): Google Reader added search.