I'm getting more and more keen on a 135/24mm combo. I just checked up on the 1.4/2x-extenders on B&H, and according to them the latest version of those extenders fit the 135, but not the 100mm L macro. The 135mm f/2.0 with a 1.4x extender would then make me a 189mm f/2.8 lens, while the 2x extender would make a 270mm f/4.0 lens. The lack of IS may be critical at these focal lengths, but if the alternative is another lens to carry around, then I could probably live with a little higher ISO. Besides, an extender could even fit in the pockets of my jacket!

Nice shot clemmb, thanks for sharing!
I'll think about something to fill in the midrange (35-50mm range). A 50mm isn't all that expensive anyway. Alternatively I could add the 24-105mm, but not untill I've accustomed myself with the primes.