Alright!

I re-installed maemo 5 using Nokia Software Updater and it worked well (except when XP told me the n900 unpluged during the setup -> start it again and OK).

That was the opportunity to enjoy the backup utilities

I got back mails, sms, applications , and even the setup of the widgets on my desktop

the only bad point is that I've lost the setting of my Msn-haze account and all contacts from it.

So far... everything's fine!