Nest has pulled the Nest Protect from sale because of an issue where the detector could unintentionally be turned off, delaying the alarm in case of a real fire. Once the issue has been fixed, they’ll put it back for sale.
If you have one of these, Nest suggested you immediately disable this feature. When they fix the issue, they’ll produce an update.
Read more at 9to5google.com.
Update: As of last week (mid-June 2014), the Nest Protect is back for sale.