Good points Jeff. I'll never let my 100-400 go because I get the reach and flexibility of a zoom. A great lens for the $$$. Sadly I Doubt Canon will ever update this lens with the new 200-400L that was supposed to be available later this year, prior to the disaster in Japan.
The new 200-400 has a built in 1.4x TC so you avoid all of the valid concerns Jeff noted about swapping TC's/lenses in the field.
It's going to be a great piece of glass for those who pony up.
That lens is expected to have a MSRP around 10k and sell for roughly $8,000.
As an aside Jeff is shooting a Nikon and currently (IMO) Nikon is handling high ISO noise better than Canon.
On the 7D I prefer to stay 400 or lower but find images clean up nicely all the way to 1600.
