Normally when a social networking app start spamming its users, it's a result of either unscrupulous third-party application developers, or scrupulous ones that are under attack by hackers.
Right now, however, Twitter is in the process of developing its own mobile web application - one that works for all smart phones - and it has been rolling it out slowly to its user base since May. Unlike the third-party applications, this is not one of those apps you need to authorize. It will be added to your account automatically if you access Twitter from your smartphone, not using some other web app developed for your smart phone.
Unfotunately, this still-in-development application is currently under a spam attack. As a result of people not even knowing they've downloaded the application in the first place, there is a lot of confusion on Twitter. Spam advertisements are being tweeted from users accounts without them realizing it until they either check their profile or a Twitter friend brings it to their attention. Because Twitter users are really sensitive about spam tweets (reading a feed on a mobile device is challenging enough without uncessary profile updates that mean nothing to anyone), some users are losing their friends and being reported for spam over it.
So what can you do? Right now, I would suggest first checking Settings-Applications on your Twitter account to see if the mobile web app has been activated, and deativate it. While you're there, you should also deactivate any other applications you're not longer using - that should be done periodically, anyway to reduce your chances of hacking.
Next, Check your profile to see if any of these spam tweets have been sent from you, and delete them.
Finally, if you want to access Twitter from your mobile device, you may want to consider using a highly-rated third-party application from your web-proider's application store - at least for a little while longer, until Twitter gets the kinks worked out of its new creation.