I'm not a Mac guy, but I found this thread on Google when looking for ways to keep the glass top from sliding around on the Ikea Mikael desk. I tried those stick-on rubber pads, but as I put my elbows on the glass throughout the day, I would slowly push the glass forward, smearing the sticky back of the pads as it moved. I thought about screwing some corner brackets on the back of the desk to give the glass something to push against, but first I thought I would try my idea of putting under the glass one of those pads that you put under area rugs to keep them from sliding around. It looks a little unorthodox, but I think I like it (and it works perfectly). Here's a pic:
Again, sorry for the ugly Dell