If I write a tweet and it contains a URL, I figured out that it does not show up in searches, regardless of the URL (I even tested out Twitter.com !) Somehow, I must have been flagged as a spammer because other URL's show up in searches from other twitter users. These two known, effected accounts of mine aren't very old (but not brand new either), and they don't have many followers at all. I hope I didn't spread the disease to the other twitter account by logging out, and then logging in with the other account to test this. Just so you know I'm not one of those in your face advertisers. my accounts wouldn't look as if they were run by a bot, and most of my tweets are not promotional.
I'm actually just talking about Twitter search. I must have done something that rubbed twitter in the wrong way, and I was probably auto-flagged as a spammer.