You should definately check all those items, but I think you'll find it is your starter. My 06 started doing that about 18 months ago. First it wouldn't turn over when the key was turned every once and a while, then it would take two or three times turning the key about once a week to get the car to turn over, finally one day it just wouldn't crank at all. Replaced the starter and all has been fine since then. Note that you need to have triple jointed elbows half a dozen different size socket extensions and, long skinny alien fingers to reach the upper bolt on the starter, and a mirror to see it, other than that it's not a tough job.