Question re: Game Center on multiple devices (iPad and iPhone)

I just bought my first iPad, and have a question ....

I have several games that are universal builds -- for example, Jetpack Joyride. I have had that game on my iPhone for a long time, and I have all of the achievements.

If I put that game on my iPad now, and connect it to the same Game Center account, what happens with my achievements? Do they merge, are they kept separate, or (worst case scenario) does launching the game on my iPad wipe out the achievements on my iPhone?

I asked this question about the Apple Store when I bought the iPad, and got conflicting answers.

Anybody know for sure?

Thanks in advance.