Let's not forget that you are able to get a replacement screen for your viewfinder. It is known as the EF-D screen and costs around $40 shipped from B&H. This screen uses grid lines that can help with the rule of thirds, and can also be handy for alignment of horizons etc.
I installed one in mine, and love it. Its not a thirds grid, but functions like one. Not to mention that I cannot remember the last time I had to straighten a photo in post -- my pics may stink, but they are level!

