Recently I was provoked by a thought of retweeting a tweet by some particular tweeter automatically, and thus begun my search for such a tool or web app. To my surprice I could not find any instead I stumbled upon how to use yahoo pipes to do that.
The way i did it was as follows
Instead of creating my own pipe from scratch I used clean twitter feed pipe and edited it to my convinience.

Firstly You need to have key word.
I took it as Guykawasaki
so for me the feed url would be
http://search.twitter.com/search.atom?q=guykawasaki as I wanted to retweet all posts by Guykawasaki

1. Add a fetch feed box and enter the url in it.

2. Next add a Regex boc to filter our the unwanted content based on regular expressions

3. Then add a filter funtion to perform filtering on basis of the above expressions

4. Connect the pipes and you are done.

Now we goto the properties of the pipe and click run it. Once we are on the results page which looks like this.

Here what is of important to us is the RSS of this pipe. Clicking it gives us the RSS link of the pipe and we are almost done.

Now what we need to do is just use twitterfeed to automatically post the updates to the required twitter account.
If unknown to twitterfeed see how to do it here.