Those people would be using the term bricked incorrectly then.

Bricking is when your device becomes about as useful as a brick, or a door stop or such. If your PC crashed and you had to pull out the re-install CDs and reinstall your OS, would you call that "bricked"? No. Because once you re-install, it's still a useful device.

You can actually brick a number of phones (including some iPhones) with a hack or a software update gone wrong. It can wind up in a state where you can't fix it without custom in-house tools. In some cases, some poorly designed devices can wind up completely unfix-able. You can brick many devices where the flash loader is in flash itself, and is typically overwritten as part of the update.

The N900 has it's flash code in ROM, and Nokia has at least 3 release images that you can download and flash directly to the device via common computers that most people have access to. You can reload that software if something happens and you'll wind up in a known state, with a working base device again. And since all the tools are local, you can download and archive them, and as long as you have another device with a compatible architecture for flashing, you can use those archives for as long as both devices survive.

The only way to brick an N900 is by damaging hardware, and with very few exceptions (eg. i2c) you can't do that via software on the N900.