I know OS x86 doesn't ask for serial during installation but that's to be expected! I haven't installed os x on a real mac so i'm wondering does Leopard, or any of the other OS X's require a serial during installation or some kind of activaction?

Not on PPC macs for sure and I doubt Intel Macs either. I think the server version of OS X does require a serial number though.

The server vesion does, not like windows though. You need the serial when you turn it on the first time during the setup. No way to get around it.

When i installed Leo on my Intel based imac, it didnt ask for Serial.. so no.

Possibly the most severe thing that Apple would ever do would be to include a "Serial Number" check, where it looks to your logic board to see if it has an Apple Serial Number. (Or look to the logic board for a "Machine ID").

The Machine ID check is done now solely to determine which systems OS X is incapable of installing on (like the G3s Pre-Firewire in the case of Tiger and G3s in general in Leopard).
They could easily implement it as a check Machine ID.

it will be easily cracked like the TPM they implemented to avoid OSX86

That is very true. In fact, one can bypass the machine ID check by editing info.plist files before starting to install. It was more the case of what could be done very simply.

They could check the hash of the info.plist every install or something, how ould you get past that?

changing the hash of the check

A lot of Apple products don't need serials....like iLife. Basically anything that you buy from their store (beside final cut pro etc) doesn't need a serial. Although, iWork occasionally comes as a trial, so it does need a serial....so yeah