I've been having this problem for a couple of weeks or so, and it's gotten slightly worse with today's viewer update. I'm running between 90 and 95% CPU usage (lots of peaks to 100%) with my avie standing in a skybox. I can leave the avie standing there for 30 minutes or more and cpu usage doesn't come down. I'm not getting any lag - in the sky not moving, 64 meter draw, disabled everything I could think of, and the cpu usage stays very high. Low, Mid or High graphics don't have any effect on this (except the pc crashes if I ramp up draw distance while the cpu is already at 100). Nothing else running on the pc except for system background and Task Manager.
Windows XP, 1 core P4 three-point-something, 3 usable GB ram, GeForce 9800 GT. My connection is ok and wouldn't explain this anyways.
An interesting thing, Task Manager floats above the SL window. If I click Task Manager to make it the active window, the viewer CPU usage drops instantly to 65 - 70%. When I make the viewer active, it returns to 90+ instantly. It does that each time.
I'm pretty sure my CPU is going to over heat.