Why does dhclient only work if its not specified in /etc/rc.conf

I have just set up a FreeBSD 8.1 in a VM and am trying to get it to do DHCP automatically. Right now, if I leave /etc/rc.conf like so:

#ifconfig_de0="DHCP"

then I can do dhclient de0 and it gets an IP instantly.

If I remove the comment so that DHCP is supposedly automatic, then dhclient de0 never gets an address (and it doesn't happen automatically either)

I get DHCPREQUEST on de0 to 255.255.255.255 port 67

Nothing else changes, what am I doing wrong?

I've also tried using sysinstall, but I have found no way to get it to write the changes to the files once I make them.....

==== Edit

Hoping to save someone else the time I spent on this, here is my rc.conf:

# -- sysinstall generated deltas -- # Mon Dec  6 11:51:13 2010
# -- sysinstall generated deltas -- # Wed Dec  8 08:28:27 2010
# Created: Mon Dec  6 11:51:13 2010
# Enable network daemons for user convenience.
# Please make all changes to this file, not to /etc/defaults/rc.conf.
# This file now contains just the overrides from /etc/defaults/rc.conf.
#REMOVED: inetd_enable="YES"
hostname="pfsensedev.intomec.com"
#ifconfig_de0="DHCP"
inetd_enable="NO"
sshd_enable="YES"

I found that I had to comment out the ifconfig_de0 line if I wanted to get an IP address assigned. If I left it in rc.conf, it was impossible to get an IP address via dhclient.

Putting the lines into /etc/rc.local did the trick

dhclient de0

I'm running this on a MS Virtual PC VM under Win7x64 (already had it loaded for XP Mode).

Thanks for the help guys.