If you need to search and replace some text throughout your weblog, you should definitely be using Urban Giraffe's Search Regex. With Search Regex you can search and replace text in all the fields shown in the picture below:
If you are replacing some text, first enter the Search pattern and press the Search button. This will show you the results, so you can fine tune the pattern to get only the results you want. You also get a set of links to view and edit the post. Very handy.