Argh, ok as a RedHat user (I used RedHat 95% of the time for almost a year), I have to chime in.
GCC offers several CPU specific optimizations. The flags involved are -mcpu and -march. Note that -march implies -mcpu. The best way to explain how these work is to use an example. I had a Pentium 4. If I compiled using -march=pentium4, GCC would use Pentium 4 specific instructions (aside from SSE and SSE2) in addition to scheduling these instructions in a manner optimized for the Pentium 4. Because P4 specific instructions aren't guaranteed to work on other CPUs, we can safely say that this is the highest optimization setting. But, if I were to copy this executable to an Athlon, it might not run. If it did, it might run poorly as the instructions were scheduled to take advantage of the Pentium 4's architecture.
Now, there is also the -mcpu option. What this tells the compiler is to generate code that includes scheduling optimizations but which does not use any special instructions available to that CPU that may not also be available to other processors. For instance, compiling with -mcpu=pentium4 will optimize the instruction scheduling for the Pentium 4. But I am guaranteed to be able to run the executable on an Athlon or a Pentium 3. Any i386 processor for that matter. It may not run as well as the instruction order has been tuned to the P4, but it will run nonetheless.
RedHat is not compiled for i386 in the general sense that most people first think. Only i386 instructions are used, but programs included in the distribution are often compiled with -mcpu=i686 to enhance performance for current-class CPUs while retaining compatibility. That way, RedHat can guarantee you can install the OS on a Pentium which is at the i586 level, or a VIA C3 or some other oddball x86-compatible CPU. For the most part, applications rarely benefit from these GCC optimizations. People have this deluded impression that GCC will spit out code that is 20% faster when in actuality you probably are lucky to get a 3 or 4% gain.
There are some components that RedHat does optimize because they are so fundamental and frequently-used that optimizations make sense. RedHat releases kernels that are compiled with -march optimizations. They also release libraries like glibc compiled with -march=i686. But in no way is RedHat daft enough, or masochistic-enough, to release software that is somehow tuned to i386 and nothing else.
I often wish the Gentoo people would realize they are often wasting their time compiling if their intention is to tweak for performance. I wish they'd realize that any performance difference between Gentoo and RH often has to do with the fact RH by default ships with a lot of running services, and a fairly large kernel. But for some reason, they've permeated this myth across the net that by compiling all your software from source for 3 days, you're bound to get a 20% improvement in speed for your text editor.
So what you have hear is the Gentoo myth applied to Apple saying they somehow used RH9 on the Dell to be destructive. I'm sorry, I just don't buy it. They probably used RH9 on the Dell because RedHat is a recognizable Linux distribution to people curious about switching away from Windows.