/shared2/sys/net/dhcp.dat

This file contains IP-addresses used by the Wii when connecting to the internet. Here is a draft C-struct:

struct
{
  u8 ip_1[4]; // Wii IP Address. 192.168.0.3 during test.
  u8 ip_2[4]; // When I was testing this, it was my computer IP address (192.168.0.46). Wierd.
  u8 ip_3[4]; // Annother IP on network (192.168.0.5). Never connected to it.
  u8 padding[20];
  u8 ip_4[4];  // Unknown: 199.183.190.001
  u8 padding_2[4];
  u8 ip_5[4]; // weird ip, started with 0x70 0x70.
  u8 padding_3[48]; // just some padding probably
} dhcp-file; 

For system menu 3.2, the file on my wii followed this structure:

struct
{
  u8 ip_1[4];		// (last) Connection 1 IP
  u8 ip_2[4];		// (last) Connection 2 IP
  u8 ip_3[4];		// (last) Connection 3 IP
  u8 padding_1[32];	// 0x00
  u8 ip_4[4];		// Unknown: 74.6.3.1
  u8 padding_2[4];	// 0x00
  u8 ip_5[4];		// Unknown: 74.13.82.140
  u8 padding_3[40];	// 0x00
} dhcp-file; 

Note: The Wii prompts for a "system update" when it checks the internet connection. Could it be the update server IP is in this file?

Could this possibly be just a DNS cache?