I blogged previously about Twitter's trademark issues:

1. Trademark Issues for Twitter
   
   
2. Trademark issues for Twitter - Part 2
   
   
3. Twitter Trademark Policy

The third post talked about an application to register "CoTweet".  Via Aaron Silverstein, I see that a company has filed an application for "Tweetup" [link]  Aaron's reaction?  "Good luck with that."  I'm not sure what exactly the company does, but the application lists advertising services or something similar.   The big issue facing Twitter is the fact that they have allowed third parties to freely use the "Twitter" mark to market arguably competing products or services (or services in the same space). 

There's also this:

Currently according to domain-search.domaintools.com there are over 3,388 registered sites that use Twitter as part of its Domain Name.

Wow.  I'm guessing that the majority of those are not registered to Twitter.  Separately, Tyson Snow suggests the possibility that Twitter may have a "Xerox problem" - i.e., "Twitter" is starting to become synonymous with micro-blogging.  (I think when most people talk about "Twitter" they're talking about *the* Twitter.  Not micro-blogging generally.)

On a loosely related note, the much hyped AT&T/Federated Media project titled "March Tweetness" was pulled because of the NCAA's complaint that the website infringed on the NCAA's trademarks.  (Twitter collaborated or consulted on the project I think?  The precise nature of Twitter's involvement is unclear.)  Either way, the micro-blogging darling faces a few of trademark issues.