I believe you earn 'temporary' trophies in the trial versions, but they don't get synced with your PSN account until/unless you buy the game.
inFamous is the only trial I have played, but since I already own the disc version (I was considering buying the DD version and trading in/selling my disc version) and have platinumed it, I can't say for sure. I did find out that the save files from both versions are compatible with each other, though (when I started the trial of the DD version, I started at the end of the good karma ending on Hard - exactly where I left off the last time I played the game when I earned my Platinum trophy). I started a new game and played for the rest of the trial time. Once it was up, I popped the disc in and picked up right where I had left off.
I downloaded the Prototype trial, but have not yet played it (I need to make sure I start it when I have the time to play the full hour).
When you earn Trophies in a trial, they are stored locally on your PS3 until you either buy the full game (Either through the store OR buying it on disk). You'll know they're only local trophies when they have the 'install bubble' around the trophy icon. After that point you buy the game, they'll instantly turn into actual trophies.
At this point you can't get rid of them, and they are not sent to Sony (as far as I can tell anyway, as they never appear on opther PS3's I've synced). You may be able to get rid of them by deleting your local account then recreating it, I havent tried this.
I agree with your assessment. Played the GOW trial and while I unlocked trophies, they were only local until I ended up purchasing the retail disc. Once I did that, they synced to my account.