From reports on other sites, it seems like Amazon is no longer creating new Mturk worker accounts. There is no official reason stated just that nobody since July 19, 2012 has yet to receive the verification email to allow them to start working on Mturk.
For those that have completed their Mturk registration, don’t panic, you are not alone. Do not try another registration for Mturk only allows one Mturk account for each worker and signing up for more than one will likely get them all denied.
Possible reasons for this? All I can offer is pure speculation. Mturk’s work base might have gotten so large that they can no longer effectively manage it. Possibly, they might be planning to clean house and by closing registrations prevents the newly suspended workers from returning. Lastly, they might be in the process of creating a new registration system that requires more proof of who you are.
So far, nothing official from Amazon about this has been reported by anybody. So nobody has any idea how long Mturk will not be accepting new workers.