I was looking into doing this with mine before it got stolen.
It is very simple. Like everyone else said, it's just like changing out the HDD in your tower. Just make sure you have everything you need in order to get the PS3 running again afterward.
There are actually many in depth sites that detail just how to do this, though I don't remember any of them off the top of my head.
In regards to Troncoso's last post...
They should give that guy an award, because it's extremely hard to 'hack' a PS3.
But installing another OS onto the system isn't really hacking. More like modding.
The PS3 in itself is just a computer with a dedicated OS. Installing another one like Linux just changes the platform a bit and allows you to do more. You are, in a sense, not using the Playstation OS anymore when you do this.
Now, of course, it allows you to do several illegal things like burn ISO's of your games to your harddrive, but still...