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Firewire and USB operate separately from the computer’s internal I/O bus (PCI in older computers, PCI-Express today). To communicate with the system, Firewire and USB devices need to shove their data through “bridge” controllers (effectively translators) that add latency and reduce performance.

Thunderbolt has great promise: unlike Firewire and USB, it’s effectively extending the PCI-Express bus over the cable directly to external devices. It should be much simpler and much faster. It’s like ExpressCard slots, but over a cable.

The simplicity and raw speed of the communication path means that it can support Thunderbolt-to-Gigabit or Thunderbolt-to-Firewire adapters. That’s enough to overcome one of the major hurdles to adopting the MacBook Air’s wedge shape in a future 15” MacBook Pro, as I speculated about in January:

It would be very difficult to fit Ethernet and Firewire ports into the sides of a wedge-shaped case. … I don’t think Apple would make proprietary tiny ports with dongles for Ethernet or Firewire, and neither can be operated well through USB, so to adopt the wedge, they’d probably need to drop both.

Now we have a better solution: the next 15” MacBook Pro can drop its Ethernet and Firewire ports and just offer optional Thunderbolt adapters.¹ These probably won’t be cheap for a while, but even if they’re $50 or $75 each, Apple would be able to safely drop the big ports from the 15” without much complaining from high-end buyers.

And since it’s effectively impossible for pre-Nehalem CPUs to support Thunderbolt safely, we can deduce that Apple probably won’t bring Thunderbolt to any product lines that currently ship with Core Duo CPUs until they can update them with the Core i3/i5/i7. It only makes sense to ship a Thunderbolt-equipped MacBook Air (or Mac Mini, or plastic MacBook) when a Sandy Bridge CPU upgrade is ready to ship with it.

So the next MacBook Air, presumably with both Thunderbolt and much faster CPUs, is going to be significantly more awesome. And that’s saying a lot, because the current Air is pretty great. And I can’t wait to see what happens to the 15” line next.

I’ve recently written a lot about laptops closing the gaps with desktops, and that’s saying a lot, because I love the Mac Pro. But Thunderbolt can bring even more of the Mac Pro’s former advantages into the laptop world.

We just need to wait and see how good the Thunderbolt devices end up being, once they’re eventually available. But I bet 2012 will be a better-than-usual year to be in the market for a new laptop.