Hi,

> I have been experiencing heat issues with my Mac Mini while running
> Xfree86.  What happens is that when I run X the bottom plastic part of
> the Mini becomes hot to the touch, and whenever I launch an
> application like Firefox, the fan goes on for a brief period of time.

Perhaps there's something maxing the CPU?

> FreeBSD/powerpc has problems with the Mac Mini as well; simply running
> it will make the fan work nonstop.
>
> Is this behavior common or expected?  Can the hardware be damaged if I run X?

Well, there are two possibilities. One is that the fan isn't going on when 
it's supposed to; the other is that the CPU is maxed. It's possible that 
the CPU could overheat if the fan doesn't go when it's supposed to, but 
you'll see crashing long before any damage will happen.

Perhaps it's this:
i2s at obio0 offset 0x10000 not configured

Is the fan under i2s?

John