Many users get "filtered out" (they prefer that term, to "banned") from Twitter's search index because they unknowlingly violated one ore more of Twitter's odd Rules that discourage spamming. Twitter does a less than stellar job of making user's aware of these Rules, and an even worse job of clarifying exactly what the rules really mean. It's all very vague. What being "filtered out" means is that your updates will not show up in any searches, hash tag streams, ete. Tweets that mention your name will show up in a search, but not those actually posted from you.