Hi Thierry!
If you have added custom widget menu options as specified in the below tutorial then I regret to inform you that there is no way to conditionally show/hide those widget menu options. When editing the manifest you will notice that the "xmlToEdit" and "showIf" attributes are not used, so InfoPath has no way of determining when these options should be shown. We have hit this problem in several projects, and it's hard to convince the UI experts that it cannot be done, but trust me. The only way to hide a custom widget menu item is to switch to another view (and therefore a different XSL).
http://infopathdev.com/howto/tutorials/default.aspx?i=f02e8631f1c342df9c49fe8103e10215
After reading over this post, I see that it is posted in the IP2007 forums, but everything I have said is from my experience with IP2003, so maybe it will work in IP2007, but my guess is that it won't. Let us know if you figure it out!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge