For everyone having cat problems in their tree, try using this stuff: http://www.amazon.com/Lambert-Kay-Boundary-Repellent-Spray/dp/B0002AQS72
I've got a three cats and a 6 foot tree, covered in lights and shiny balls but after using that stuff everyday they won't get near it! Better improvement from last year when they knocked it over everyday.
Edit: Forgot to mention it smells like absolute shit for about five minutes but then the smell goes away completely.