Akubichan and Marcelo, is it possible that you used a prefpane based tool or command line(Terminal) hacks to manage your Launchpad icons?
If so, probably your Launchpad database is corrupted, that's why it gets reset to the default state when you log in.
Basically, Launchpad validates the database every time you log in.
Launchpad will:
- Remove all icons that link to nonexistent apps
- Create icons for all newly installed apps
- tries to fix corrupted databases - if it can't it simply deletes it and re-creates it with ALL icons.
Check out this post for more info.
Also, there's an OS X lion native app that helps you restore your database or manage icons - without corrupting your Launchpad database.
I only tried the free version so far and it's way better than the Air or prefpane apps out there.