There, I've whetted your appetite for buying a 70-300L, a 300 f/4 IS AND a full frame body!!You did. I was actually going to upgrade to a 6D, but the thought of learning something new threw me off as well as a new "normal" lens, etc. Plus, results with my T1i are really extremely good so I figured I could throw some dollars towards a great telephoto. My tele-primes really shine for indoor sports - my daughter's an "extreme" volleyball player (delusional sports-dad alert), but I can definitely use a tele-zoom for my son's outdoor sports, band, etc.
The 300 prime or 100-400 seem like a good move for several reasons; one of which is the same size polarizing filter that I already have - of course, an expensive one. I am a "prime"-ho so maybe I'll rent a 300 4.0 IS and buy the 100-400II when it comes out. That might be the move and bring just my normal zoom and 300.