edit:
omgnowai, I didn't see your post - are you on a Mac or on Windows?

For the first time since I bought the game, my Launcher appears to be working as it should, so I did some testing of Sims3Pack files from various sites.

I can install store-bought content without a problem - as long as I re-download it through the store. I can also install Sims3Pack files from other sources (ATS and TSR) without any problems. They all show up, have the appropriate icon and work as they should.

The only problem I have is with recompressed files that I got from sites like MediaFire. They get installed by the launcher and then kicked out by the game. When I exit the game and check the launcher, the files show up as uninstalled.
The EA pre-order bonus textures all work fine, but the other pirated files don't. Store stuff that I bought and then recompressed and archived on my computer gets kicked out, while the same file re-downloaded from the store works fine.

It does seem as if the game recognizes the pirated stuff and uninstalls it at start-up. I am not sure whether that has anything to do with the method of compressing or if it uses something more sinister to check your installed items against your purchase history...