I've heard some reports that under the newest Mac OS (10.5, codename "Leopard"), NXT-G will run fine, but is not able to update the brick firmware. I personally upgraded to 10.5 just about a week ago, and have had no problems with NXT-G (or anything else really - it was an amazingly painless installation)... but I've not tried to update the FW under this system as yet. If any of you Mac users do have a problem with this, please let us know, and when we hear more about this (one way or another) we'll try to post an update.
For now, if you do have this possible problem, just make sure you have access to a version of NXT-G running under something other than Leopard. After all, the rest of it seems to work wonderfully. Now, I've got to go see if I can verify this and brake my NXT...
Update, 9 Nov 2007 - I've verified this on my system. Under Leopard, the FW download process stalls out while preparing the NXT. I could take this newly-bricked NXT to a second Mac with OS 10.4, and repeat the process to restore the NXT to full functionality with the 1.05 FW, so as long as you have a back-up, this is not a critical problem. But if your only computer is a Mac running Leopard, it appears you can not update the FW currently.
Brian "Leopard bytes NXT" Davis