So I went to the San Diego Wild Animal park last saturday and gave my fiancee my 50D and 70-200. We're walking aroud and she tells me the lens is acting funny. Turns out she's right.
Whenever the camera is powered on, with the lens pointed anywhere but straight down, the IS FREAKS OUT. So badly that 90% of the time you can't see through the viewfinder. So we spent the day trading my 100-400. After a few days of fuming, I put in a web claim on Wed. Thurs I ship off the lens.
Monday they received it, Tues they repaired it (Under Warranty) and I got it today. Not bad.
For future reference (It may help someone out there)
Check IS for excessive noise during function. Lens Won't enage focus.
Your product has been examined and it was found that the image stabilizer assembly had suffered some kind of shock damage the camera shake prevention feature did not function. The image stabilizer assembly was replaced and product functions were confirmed, Best Focus adjustments, inspection and cleaning and mechanical adjustments were carried out.
I bought the lens < 1 year ago, and they repaired it under warranty!
The lens has NO marks on it, and I can guarantee it wasn't dropped or banged around..
If anyone has a problem with this lens and the IS freaking out and making tons of scary noises unless you reorient the camera/lens just right.... this may be your issue.
Hope it helps someone!