Hi Yann,
I had exactly the same problem as Fireproof, so if you've upgraded a 3g to iOS4, you have my sympathy.
Greg Jones kindly found me a solution, you can find it amongst my anti-Apple ranting in this
thread.
However, be warned, I lost all the data associated with my Apps and settings. So you might want to wait for iOS4.1 in which, it's rumoured, Apple will be addressing this issue.
If you have the OmniFocus for Mac program, another solution that some Members have found useful is
here.
Hope that helps.