Okay the way the Trophy system works is that you have bronze, silver, gold and platinum Trophies, which Trophy you get is based on the difficulty of the requirement. Platinum trophies are only attained if you get ALL the others in said game, for ultimate bragging rights. Only full retail games will have platinums, the only downloadable games with them are Warhawk and the upcoming WipEout HD (gonna be dope!) Getting Trophies is no cakewalk, that shit is hard, you gotta bust your ass to get a gold. Which is good cause it’s gonna take skill to get them and it’ll set the Almighty’s apart from the clowns.
The developers, not Sony choose what the requirements are for each Trophy in said game. We have a Gamer level, which takes into account the number of Trophies you've earned and displays your progress via a percentage bar towards the next level; developers can customize the Trophy achievement art to fit their games. It keeps everything organized and simple, you can look at someone’s Gamerlevel verses how many games they play and you can tell if he’s a hardcore or light gamer.
Here's the bad news, you'll need to earn Trophies in games that are patched to include them even if you've already completely beaten the game. For example, I beat Uncharted before it received a trophy patch, I’ll have to play through it again, in order to earn its Trophies.
All new PS3 games going forward will have Trophy support and that’s where the competition will start to heat up. We’ll see how much of a gamer everyone is just like on XBOX with their Achievements system. Check this list to get yourself situated:
http://www.thesixthaxis.com/ps3-trophy-faq/