Not true. I joined the avr-chat mailing list a while ago, as well as the gnuarm list, and on both lists I found a number of experienced developers who do all their embedded work under OS X (and sometimes other Unices such as Solaris). It can be done, and works great once you get the tools set up. There's just less hand-holding to get you started (which is ironic on some level).
Probably true, but I'd prefer gcc over commercial compilers anyway.
But I see from your list that you already knew this for ARM and AVR -- but those are the chips I'm mainly interested in anyway. If I can't use, say, a Rabbit microcontroller without Windows, then I simply won't buy a Rabbit. There are plenty of more open alternatives.
Cheers,
- Joe