Rocking 8400 Base

I bought a Dimension 8400 back in August and I didn't take much notice at the time but the case doesn't sit flat on the desk. The front of the case wobbles about 3 millimeters from sise to side. I didn't take much notice at the time and just stuck a drink coaster under the front of the case to stop it rocking. Over the last couple of days I thought this is ridculous and phoned Dell and told them about the problem. They said if I had of told them about it when I first got the machine they would have sent out a new one. Now all they are willing to do is send a tech out to see if he can fix it, which is fine by me if he can. But if he can't can I still demand a replacement unit, after all the machine is warranted for 12 months. Anyone else had the same problem with the rocking of the case? What do people think about the replacement if they can't fix it. Any feedback appreciated. ricardo