Coincidentally, I noticed play in the swinging arm of my 06 AE a few days ago, and it has also done just over 8000 miles. The rear end has recently been apart and rebuilt because I had the dealer investigating a road speed-related vibration. As this same dealer totally cocked up the simple task of refitting the front wheel, I had my suspicions and decided to have a look at the rear end myself in spite of the fact that the bike is of course under warranty.
I got it apart today and have yet to put it back together but here's what I discovered. The locknut was not torqued and only slightly more than finger tight! Wiith the swinging arm out of the bike and the spindle and bearings assembled I couldn't detect any play in the bearings. It's a peculiar design but what I think is happening (with my bike at least) is that with the locknot loose, the whole assembly (swinging arm, bearings and spacers) was moving in the frame at the left hand side. To be honest, I can't quite get my head round how the whole thing is meant to work. The spindle is threaded into the frame at the left side so it's not yet clear to me how the movement occurs. I am guessing that the whole thing has to be torqued to 115 Nm to create sufficient friction with the frame and prevent movement. Perhaps some clever bugger out there can explain what Yamaha are up to with this design?
I'll confirm my diagnosis or otherwise in a couple of days when I have reassembled with the correct torque.