It seems that Spam accounts have adopted a new way to appear valid, (or I just noticed that, maybe it's not that new), by copying valid tweets from different users before sending the usual "here are my naked pictures".
Take a look at one example:
Click to view my naked profile http://xrl.us/be4juv
less than 20 seconds ago from web
@onlymehdi He is not alone, God is there, keeping him strong, his heart is full and our love/prayers sent to nourish him. #iranelection
half a minute ago from web
Follow Friday! @DavidArchie, @ddlovato, @Shontelle_Layne, @ muckytown @TheRealJordin, @TheDannyNoriega! :D
less than a minute ago from web
RT @TheTorchTheatre: Sunday! @bookmans sponsors The Improvised Bookclub! This month: Harry Potter & The 1/2 Blood Prince! @Space55, 7pm!
1 minute ago from web
The most recent tweet contains a spam link, the other ones are copies from tweets a few hours ago, since the user starts mass following before the spam tweet is posted, you will not recognize that immediately.