mkinitrd-6.0.75-1.fc11
dhcp-4.1.0-5.fc11

mkinitrd/nash/network.c:
if (execl(DHCLIENT, DHCLIENT, "-4", "-1", "-n", NULL) == -1)

http://people.redhat.com/wtogami/temp/f11-netboot-failure.png
It seems that -4 is an invalid parameter, because dhclient 4.0.0 and 4.1.0 both print syntax help when you use it.

dhcp-4.1.0-5.fc11
=================
dhclient -v -1 -n eth0
[outputs the syntax help]

dhclient -v -1 -n
[snip]
execve (/sbin/dhclient-script, ...): No such file or directory
No broadcast interfaces found - exiting.

Bringing up the eth0 interface manually before dhclient makes no apparent difference in dhclient's behavior.

dhcp-4.0.0-34.fc11
==================
dhclient -v -1 -n eth0
Sending on   Socket/fallback
receive_packet failed on eth0: Network is down
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 5
send_packet: Network is down

ifconfig eth0 up
dhclient -v -1 -n eth0
[This works!  Although it complains about dhclient-script not existing.  nash segfaults during parsing its output though.]

Bugs
====
1. /var/run should be created by mkinitrd.
2. -4 parameter to dhclient is invalid.  dhclient's man page is wrong, or bug?
3. eth0 should be brought up before dhclient?
4. dhcp-4.1.0 dhclient is broken in some way compared to 4.0.0?
5. nash segfaults because it cannot handle parsing the output of dhclient's output.  (Or maybe the output of dhcp 4.0.0 is different from 4.1.0.)
6. dhclient shouldn't complain about missing dhclient-script??