You can now do this through the web interface of Twitter.com
When you search via the webapp, the results will show a filter of users between
Top / All / People you follow
By default the results will come back filtered by Top.
Select People you follow to filter search results accordingly.
You will also notice the URL change/update to the following (on an example search of "watching"):
The following is the part that says you're filtering results to only accounts you follow: