Thank you, for your very competent advices.

Bluesmirf was also my preferred bluetooth module, because it appeared in a tutorial for arduino with n900.

But it was to expensive to me.

So I've bought this one:
http://cgi.ebay.com/Arduino-Serial-P...item2563f92b8a

It seems to have the pins already fixed. I don't know if the pins are compatible.

Do you think it could work?

(The digital compass has an explicite instruction for arduino mini pro 3,3v)