I don't know why they dropped FireWire 400, but it doesn't really surprise me given various comments on these boards. The powered FireWire 400 connector design is apparently too easy to torque (or worse, insert backwards), causing power to hit critical electronics, thus resulting in equipment damage (logic board failure, device failures, etc.). A few years ago, Apple put in self-resetting fuses in an attempt to fix the port shorting problem, but that also limited inrush current and caused a bunch of non-spec-compliant devices like the Presonus FireBox to stop working off bus power. There's definitely good reason to move to the FW800 connector exclusively. It's just a fundamentally more robust connector design.
If you are interested in buying a MacBook with FireWire, there is some good news. At least for the time being, the previous generation white MacBook is still available from the online store. I don't know how long that will continue, but that does provide a 13" FireWire-capable design at least for now.
My advice to everyone concerned about the state of FireWire would be to send
feedback. Let Apple's hardware team know what you think, and give a thorough explanation of why FireWire is important to you. If the decision is going to change your future purchasing, let them know how and why. Remember that when the MacBook Pro came out without FireWire 800, lots of pro users screamed about it and Apple listened and brought the port back in the next model. It certainly can't hurt to try.
Better yet, go out and buy one of the white MacBooks through the online store. If Apple sees enough people choose the FireWire-laden white MacBooks over the new aluminum MacBooks and sees enough feedback to correlate that trend with the removal of FireWire, that will likely get somebody's attention. Even if it doesn't get FireWire back on the MacBook, it will at least provide ammunition for people at Apple to argue that there is sufficient demand for a 13" pro model.
Worth a shot, anyway.