I did have the same issue with my Blackberry 8800 flexing on the sides aswell on the bottem base.
I had called a few Cingular Stores round the area, asking them if they were getting customers having the same issues. In conclusion all the Cingular Stores were consistent, and had informed me to take it back to the store I had purchased the BB. I recieved a new device.
-Currently the new replaced BB 8800 is not flexing anymore, but I believe the real issue thats contributing to the device flexing is due to the battery back cover (make sure there is not dust, nor lint in between the insert)
When I first encountered the issue, I placed my BB (backside) on a warm surface by the sun, which had expanded the plastic slightly enough which fixed the issue. At the time I just wasn't sure if it was the device, so replacing it did fix the issue.
__________________ Cingular Blackberry 8800