Chris, here's an answer I found for you on an Apple discussion forum:
<<<Hi,
The Cube chassis can get stuck in the case because the Airport card or other internal piece is out of alignment. This issue is brought up on Cubeowner.com from time to time and the usual method of removing the chassis is using a long thin blade to run down the inside of the case (between the chassis and plastic case)to try and find the obstruction. A cake frosting knife is ideal for this.
A certain amount of tension by pulling on the handle, inserting the knife and trial and error usually does the job.
There is no easier way that most have found to do this.
Just go slow and it will eventually pop out.
Regards,
Dave>>>
http://discussions.apple.com/thread.jspa?threadID=1062562