I have a late-2008 unibody that is getting up in age, and mine has just done this on the left side too. In fact it was just sitting on the table closed last night when I saw and heard it come loose. It has never been dropped, and I strongly disagree with the people who seem to insist that drops and mistreatment must occur to cause this problem. The bottom parts of the display assembly near the clutch experience torsion during normal opening/closing. Add to this that these models emit most of their heat from the left rear fan, and I think you have the formula for a pretty simple heat and mechanical fatigue problem.
I am going to repair mine soon, and I expect the similar problem of the frame coming unglued from the housing and possibly being cracked. If this is the case, my repair will be to rough the inside of the housing if I am able to access it, apply JB-Weld and clamp overnight. JB-Weld is a metal epoxy and bonds particularly well to both aluminum and plastic. For those who have had failures with other epoxies, I would suggest either JB-Weld or LePage Metal Epoxy as a proper adhesive for this job.