I was trying to recreate my BB Playbook development environment (Flash Builder 4.6 & BB PB SDK) on my brand new laptop and while trying to register my signature keys I ran accross the following error:
"Unable to register client 'xxxxxxxx' because there are no more registration attempts. If you have already registered with this server, then you must contact RIM to register additional users."
This happens when you have either unsuccessfully tried to register the keys more than 5 times or after you have successfully registered the keys (my case). So the 2 CSJ files that RIM sends by email (RDK & PBDT) are single use and once registered they can not be reused. However, once you register them, 3 files are placed on your local machine:
1. Your certificate file (*.p12)
2. A file called barsigner.csk
3. A file called barsigner.db
All 3 files should be in %HOMEPATH%\Local Settings\Application Data\Research In Motion or a similar path.
If you need to develop on a new machine, just copy the 3 files over to the new machine in the same folder and you should be good to go. It's also a good idea to back these files up. You can read a tutorial on how to do that here: http://supportforums.blackberry.com/t5/Testing-and-Deployment/Backup-and-Restore-BlackBerry-Code-Signing-Keys/ta-p/837925 or you can also do it strait from the Flash Builder interface.
Requesting new keys should be viewed as last resort especially if you already have deployed apps in AppWorld because you will not be able to release updates for those apps using the new keys. If this info doesn't solve your problem, you can also try emailing devsupport[-at-]rim.com