Brick

The term "bricked" refers to nonfunctional device states. Though nominally meaning that the device is permanently damaged, in practice it includes conditions which range from trivially recoverable (a failed update) to completely unrecoverable (in certain cases involving damaged baseband memory). A phone may be referred to as "bricked" if it will not boot, will not respond to input, will not make calls, etc. Early unlock solutions frequently resulted in partially-bricked phones after firmware updates were applied. Generally speaking, the iPhone is quite difficult to brick permanently via software, and in almost all cases the damage can be reversed.

Irreversible Software Bricking
The only way to irreversibly brick your iDevice in software is to flash an invalid baseband bootloader, provided it has a baseband. All other bad flash scenarios are recoverable some way or another. It is impossible to brick your device simply by jailbreaking it, since DFU Mode will still be available to recover from a bad flash.

If you purposefully erase / zero out your NOR, then you will even have trouble doing a DFU restore because important information from the SysCfg section will not be available.

Software Bricking with wrong version
Another recent way to brick the baseband is by installing baseband 06.15.00 on an incompatible device. redsn0w has an option to install this baseband on the iPhone 3G or iPhone 3GS in order to get a baseband version that is unlockable with ultrasn0w. This is a nice way to get an unlock, because the iPad, the iPhone 3G and the iPhone 3GS all share the same Baseband Device, but the iPad has a newer version number in its baseband. That way people can actually downgrade by installing a higher version (there are no APTicket checks in these devices). This has known side-effects, like losing GPS functionality (this baseband comes from an iPad which has GPS not or differently implemented).

Now bricking a iPhone 3GS baseband is possible with this method. In fall 2011 Apple replaced the NOR flash. It is not clear if this was done intentionally to prevent this method. Previous type was 36my1ee and they changed it to 36my1eh, 36my1eg. (Please note that there was no switch to Toshiba baseband devices as some sources on the Internet still tell.) These new NOR flash chips seem to work with the newer baseband versions in the iPhone 3GS, but are not supported with the old 06.15.00 baseband. Therefore installing this version will brick your device if you have a new NOR flash, as you (currently) cannot go back and install anything else. To check before installation, check the version number, as it reveals the production year/week in the digits 3...5. Week 34/2011 appears safe so far, 35 seems iffy, 36 seems iffy, 37 is not safe. Or open the device and check the chip type.