I found quite a solution,
but I'm afraid it won't work for anyone but me.
While googling to find a solution, I found a post, dating September 2004, describing the same problem I had. I just wondered if things would work better with a more recent kernel. So, I updated my kernel to a 2.6.11 i686. (I was running a 2.6.8 i686). While I was at it, I decided to update my Debian from a Sarge 'stable' to a 'testing' version.
To cut a long story short, while updating my system, I discovered weird things regarding my webcam, which is an old Philips Vista Pro. Believe me or not, but since I plug my webcam, I'm losing USB recognition!
Well, as I'm not using my webcam under linux, unplugging it is not a problem. Yet, it's not what you can call
a solution.
Maybe some of you also have a rather old USB 1.1 device which can create a bug.
Honeslty, I can't remember if I tried booting my 2.6.8 i686 kernel with my webcam unplugged, so I don't know if my system update helped or not. Maybe an update isn't a bad idea.
Anyway, if anyone knows why my webcam acts so weirdly on 2.6 kernels, please, let me know