The issue of bricking came up in the early days of iPhone unlocking when the unlocking software rewrote sections of the baseband processor (there are two iPhone processors, one for the display and GUI and OS and the other called the baseband handles the communication to WiFi, bluetooth and cell towers). Then when Apple would patch the firmware code in the baseband to fix issues, the new changes didn't work with the hacked changes and the phone would "brick" and become completely inoperable... until Apple later released the 2.0 firmware release which completely updated the baseband and unbricked all of the 1.2 firmware bricked phones..
The newer unlocking hacks use Apple's ticketing systems running in the OS kernel to check the unlock/locked state and are much closer to method that Apple uses to check the locked/unlocked state. They are not touching baseband firmware and are effectively background processes in memory. For more details, read Geohotz blog:
http://iphonejtag.blogspot.com...lesn0w-technicals.html
To use computer terms instead of cell phone terms, the original unlocks messed with the BIOS, the newer ones use background processes in the operating system. In the old system, if anything went wrong then the BIOS was corrupted and the iPhone wouldn't boot. The newer methods mess with the OS and thus if anything goes wrong, you can just reload the operating system - which is the firmware from Apple - and everything is fixed.
Which is a long-winded way of saying - you can't "brick" your 3.0.1 iPhone by using the Redsn0w/Ultrasn0w unlock. So whomever was saying that was wrong.
As far as upgrading to 3.0.1, I wouldn't bother. All they fixed was that SMS hole - that was the only thing at all changed. Unless you are worried about that SMS hole - and now that it's patched on 99% of the iPhones in existence you are pretty safe - I would just leave things alone and keep using 3.0.
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