From looking at the code it seems like server-id should really
be named server-IP.

---

Index: dracut/dracut.cmdline.7.xml
===================================================================
--- dracut.orig/dracut.cmdline.7.xml	2011-12-27 09:02:31.321211186 +1100
+++ dracut/dracut.cmdline.7.xml	2011-12-27 09:02:54.261623619 +1100
@@ -453,7 +453,7 @@ with a valid DHCP root-path.</para>
         </varlistentry>
         <varlistentry>
           <term><envar>ip=</envar><replaceable>&lt;client-IP&gt;</replaceable>:
-              <replaceable>&lt;server-id&gt;</replaceable>
+              <replaceable>&lt;server-IP&gt;</replaceable>
             :<replaceable>&lt;gateway-IP&gt;</replaceable>:<replaceable>&lt;netmask&gt;</replaceable>:<replaceable>&lt;client_hostname&gt;</replaceable>:<replaceable>&lt;interface&gt;</replaceable>:<replaceable>{none|off}</replaceable></term>
           <listitem>
             <para>explicit network configuration. If you want do define a IPv6 address, put it in brackets (e.g. [2001:DB8::1]).
Index: dracut/modules.d/40network/parse-ip-opts.sh
===================================================================
--- dracut.orig/modules.d/40network/parse-ip-opts.sh	2011-12-27 09:01:07.000000000 +1100
+++ dracut/modules.d/40network/parse-ip-opts.sh	2011-12-27 09:02:54.261623619 +1100
@@ -7,7 +7,7 @@
 #
 #       ip=<interface>:[dhcp|on|any]
 #
-#       ip=<client-IP-number>:<server-id>:<gateway-IP-number>:<netmask>:<client-hostname>:<interface>:[dhcp|on|any|none|off]
+#       ip=<client-IP-number>:<server-IP-number>:<gateway-IP-number>:<netmask>:<client-hostname>:<interface>:[dhcp|on|any|none|off]
 #
 # When supplying more than only ip= line, <interface> is mandatory and
 # bootdev= must contain the name of the primary interface to use for