For some time, I googled and hunted for a solution that would allow me to change the subnet that Internet Sharing (on the Sharing System Preferences panel) uses when sharing onto an Ethernet network -- the default being 192.168.2.0. In my case, this also happens to be a subnet used by my corporate VPN, which is not too convenient.
The solution for Leopard turned out to be extremely straightforward, and can be found in the unix help file for InternetSharing -- just type man InternetSharing to read it in Terminal:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>NAT</key>
<dict>
<key>AirPort</key>
<dict>
<key>40BitEncrypt</key>
<integer>1</integer>
<key>Channel</key>
<integer>0</integer>
<key>Enabled</key>
<integer>0</integer>
<key>Extreme</key>
<string></string>
<key>NetworkName</key>
<string>pimpedwifi</string>
<key>NetworkPassword</key>
<data>################</data>
</dict>
<key>Enabled</key>
<integer>1</integer>
<key>PrimaryInterface</key>
<dict>
<key>Device</key>
<string>HUAWEIMobile-Modem</string>
<key>Enabled</key>
<integer>0</integer>
<key>HardwareKey</key>
<string></string>
<key>PrimaryUserReadable</key>
<string>HUAWEI Mobile</string>
</dict>
<key>PrimaryService</key>
<string>C42284CE-6803-42A7-9F54-627DB90321DF</string>
<key>SharingDevices</key>
<array>
<string>en0</string>
</array>
<key>SharingNetworkNumberStart</key>
<string>10.168.2.0</string>
</dict>
</dict>
</plist>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090510120814850