This issue has come up in the past ~48 hours or so after a dev added a theme plugin and updated plugins.
Ec2 plugin now states that instance cap has been reached and will not provision any further instances.
Currently no slave instances exist (all terminated several hours ago).
Plugin has been disabled and re-enabled.
Cloud config removed and re-created.
Cap has been increased to 50 (normally 10)
All without success.
Log output as below
02/10/2013 9:48:03 PM hudson.plugins.ec2.EC2Cloud addProvisionedSlave
INFO: Total instance cap of 10 reached, not provisioning.
02/10/2013 9:48:13 PM hudson.plugins.ec2.EC2Cloud addProvisionedSlave
INFO: Total instance cap of 50 reached, not provisioning.