iPhone 3.0 Beta 2
With the release of iPhone 3.0 Beta 2, which automatically unlocks tethering as well as fixes a few other bugs with Beta 1 that were annoying me, I finally got around to upgrading.
However, during the process of the upgrade (I selected Update instead of Restore this may have been what caused it), an error occurred. Fearing I may have just bricked my iPhone with a firmware changed, I quickly ran Restore, only for it to fail with an Error 1611, or maybe it was 1602 or 1604.
Searching the interwebs, I found various people who had the same problem with earlier version of the iPhone firmware, and one recommended putting the iPhone into DFU mode... BIG MISTAKE! It turns out that MacOSX doesn't recognise an iPhone in DFU mode, treating it like an iPod and giving Error 2001 messages. But not only that, but I was stuck in DFU mode (which btw is a black screen with no way of knowing if it is turned on or not).
The only solution at the time I could find around this was a piece of 32bit Windows software, which wouldn't work with my 64bit OS. Luckily I came by some instructions the next day, which said to exit DFU mode, all you have to do is hold power and home for 10 seconds. I don't know if it was this however, since I think my iPhone had run out of power, I might have just been turning it back on.
To fix the original problem, all I did was delete the ipsw files in "/<user>/Library/iTunes/Device Support" directories. Restore didn't work until I manually Restore using Firmware 3.0 Beta 2 (probably something gets changes which is why you can't downgrade through normal means).