Vincent, have you made another Debian installation on the same system since your original installation report (9 Mar 2007)? Did you experencie a similar issue? If you have not, would you take the time to boot a recent build of d-i (netboot mini.iso will do) and see if the problem is still there? (As network configuration is done pretty early, you don't need to go really far in the installaion process.)
I did get it installed, but it has been running in production and I can't just randomly reboot it to test this.
I tried to reproduce with a Dell 2950 and a couple of recent d-i images. The first was a complete failure, see #491813. The second was with Lenny beta2, and that fails because it appears that the plug has yet again been pulled on the firmware for Broadcom ethernet cards. I wish that Debian would make up their minds on this. So I give up for now. Try me again in a week if you want to pursue this, I may be in a more responsive mood. Vince