For those of you that didn't know, Twitter quietly "opened the fire-hose" to all of its data to google a few months ago.
Just having a Twitter account is not enough, just like Google used to have, it has a sandbox and before you can start tweeting external links., you have to get out of that sandbox.
Twitter SEO Academy - Groundbreaking!
Once you do get out of the sandbox, I think you'll be surprised at the results if you tweet from non-spam accounts.