1.1 Framework:
StringBuilder sb = new StringBuilder();
string strHostName = Dns.GetHostName();
IPHostEntry ipEntry = Dns.GetHostByName(strHostName);
IPAddress[] addr = ipEntry.AddressList;
for (int i = 0; i < addr.Length; i++)
{
sb.Append(addr[i].ToString());
}
string myip = sb.ToString();
2.0 or later framework
StringBuilder sb = new StringBuilder();
string myHost = System.Net.Dns.GetHostName();
System.Net.IPHostEntry myIPs = System.Net.Dns.GetHostEntry(myHost);
foreach(System.Net.IPAddress myIP in myIPs.AddressList)
sb.Append(myIP.ToString());
string myip = sb.ToString();