Not desiring their brand to go the way of Xerox, Aspirin and Kleenex (which lost their trademark status in the US), the folks at Twitter wrote a post linking to not only their new logos, but also highlighting the rules when it comes to using “tweet” or “twitter” in a product or service.
We understand that you want your application or product that enhances the Twitter experience to be identifiable as part of the Twitter ecosystem. This is important to us too, but Twitter is also the name of our service and company, so we’re cautious about potential confusion. [...]
Use Tweet in the name of your application only if it is designed to be used exclusively with the Twitter platform. (Twitter Help Center)
If Twitter were to attempt to enforce their trademark rights today, they would not only have to go after TweetDeck, but also Twitter for Blackberry as both applications connect users to Twitter and Facebook (note: Twitter for Blackberry is supported by RIM, not Twitter).
Fortunately for everyone Twitter isn’t the type of company to go after complimentary services or individuals unintentionally misusing their brands, although developers would be wise avoid using Tweet and Twitter in their applications (especially those dependent upon Twitter for their business).
Twitter also listed a few other rules regarding how to spell Twitter (they’re emphasizing the capital “T”), as well as asking companies not to merchandise public tweets of users without their express permission.
Surprisingly despite owning the trademarks to both “Twitter” and “Tweet” in Europe, the company has been unable to secure rights to the latter in the US (which is owned by Paul Rosario) who fortunately for the company operates in an unrelated industry (ironically one dealing with birds instead of code).