If we believe the information currently circulating on the twittosphere, and later confirmed by the hacker chpwn, Apple would have radically suppressed access to Saurik's servers (Cydia's boss, the pulled applications manager); these so many “hosts” by the Amazon group.
Since the iPhone 3GS, Apple had taken steps to sign only its latest firmware (version 4.3.2 currently). The ultimate goal is to prevent any restoration of your iDevice to a previous version.
Saurik, in order to counter this protection initiated by Apple, offers the backup of these famous signatures SHSH on its servers. This conservation of signatures allows everyone to install the firmware version of their choice, which generally contains known vulnerabilities, and permeable desired for the tools jailbreak usual.
Pending the first reactions from Saurik, and if this ultimate blockage is confirmed, it will no longer be possible to use its servers from iTunes to restore its iDevice to a version lower than the one currently available from the Cupertino company .
Update April 20, 2011:
In a brief press release on his twitter account, Saurik explained that this was a synchronization problem on his servers, linked to a basic code preventing the registration of the most recent SHSH signatures. This problem was solved by him.
More fear than harm in fine. Saving signatures is therefore always possible, via Cydia, or even via TinyUmbrella software.
We discuss it on the forum.