I don't know if this was already discovered by some of you, but I've seen some questions about this, so I thought I'd share my discoveries in the form of a HOWTO since it worked for me. What I tested here for now are two adaptors, one for PS/2 Keyboards and Mouse to USB and a PSX/PS2 Controllers to USB.
Of course the requirement to make the PSX/PS2 controllers to work is a HID compliant USB adaptor. Why a HID compliant ? You don't need any drivers. As the PS3 don't take any drivers, you need to have something that do not requires drivers and configuration to make it work. I tried one of my other controllers that I always need this and that driver and configure it before playing, and it didn't work for obvious reasons.
Where can you get those ? Well, http://www.play-asia.com/paOS-14-71-6m-49-en.html this is probably your best bet online. As in store, I don't know in the US where you can buy those, although I do know that at "The Source" (which is Circuit City, I guess) here in Canada you can find some of those, usually for only one player, for 15$ CAD.
Alright, to make them work, you don't need to use the PS button, UNLESS, one game requires it, or to return to XMB. First thing you have to know, the first controller to be plugged in will be the player 1. What does this means ? If I plug my USB Adaptor first without pressing the PS button, then this will become player 1, then if I press the PS button on my PS3 controller, this will become player 2. Note that you can still use the PS button in game to come back to XMB even if the controller is player 2. So, if you'd wish your SIXAXIS controller to be the player 1 controller, you'd press on the PS button and wait for the player 1 led to be lit before plugging your USB adaptor in. As simple as that.
To work, PSX Dual Shock and PS2 controller NEED to have the Analog light turned on, or else the directional control (analog and D-Pad) just won't work. What happens now if you have, like me, an adaptor where you can plug two PSX/PS2 controllers ? Well, if you plug it first, your SIXAXIS will become player 3. You can't make one PS2 controller player 1, then SIXAXIS player 2 and the other PS2 controller player 3.
1. You need a HID compliant USB Adaptor for PSX/PS2 Controller
2. The first controller to be plugged in will be player 1 (or player 1 and 2 for dual adaptors)
3. The PS button can still be used as being player 2 or 3.
4. If you wish to have the PS3 controller first player, you need to press the PS button BEFORE plugging your adaptor in.
Alright, now about keyboard/mouse. If you don't have any USB keyboard or mouse, and for some reason you can't affort or you don't want to buy one, you can always get one of those PS/2 (not to confuse with PS2 (Playstation2)) Keyboard and/or Mouse adaptor for about 20$, again, at Circuit City (The Source in Canada). This uses the same principle as above. Plugged first, they become prioritary first players controllers, while if the SIXAXIS is started first with the PS button, they'll become second player and so on.
Interesting thing, for people who haven't tried it, XMB can be controlled with the keyboard, which ease the task of webbrowsing or entering any kind of text (wish I knew that the day I created my Playstation Network account -_-;;). The mouse can be used for webbrowsing or in the Playstation Store as well.
Anyway, I hope those work for you and that this little piece of information will help you ! Enjoy !