From http://forums.atlassian.com/thread.jspa?messageID=257276266#257276266 -

Bamboo 2.0 Beta4 does not seem to accept a new P4 Workspace name once the build plan is created. I have tried to enter a new exisiting workspace name when creating a new build plan as well as when updating an existing one and I get an error saying "Client was not found on the perforce server". It might work if I restart the Bamboo server but only the first time. Is this a known issue with the Beta4 build? Is there a work around for this?