TwitterHawk has undergone some scrutiny since it is able to search Twitter for specific keywords and locations and send automated tweets to those users. As you! From their site:
Let’s say you just opened a new coffee store in Queens and wanted to let people know about it. As part of your advertising efforts, you could setup TwitterHawk to search for things like “coffee near:Queens within:8mi” (of course you could simply search world wide if you are global).
They also have a statement on the site from the founder with TwitterHawk’s view on if they are spam or not. I think if used correctly, it isn’t really spam. Why? because no one says you have to pay attention to @replies sent to you. If you received them non-stop I would be concerned, but it looks innocuous for now.
I think we will see more of this as people explore opportunities on how to use Twitter as a service, and not just a micro-blog. I wonder if Twitter could capitalize and charge these types of users?
I will say their page source is one of the best documented and commented sites I have seen in some time.
UPDATE: Another opinion can be found here from TwitTip