Phil Hands did some stuff for etch / debian-installer to try to help out with this. This is the auto-install udeb. Most importantly is trying to get all the network information on the command line.
Read it here
If I keep having problems with this, Ill probably start using DHCP reservations instead of static addressing. You can even push the url out via dhcp. Ive also gone back and shortened my url hostname to d.example.org to gain a few more characters. I probably could just recompile the kernel to support more than 256 chars on the command line as well.