VPN mode
iPhone 3G screen shot in VPN mode
If you can log on to your server with a computer using VPN (Virtual Private Network) you can also connect to a network using VPN on your iPhone. The VPN will allow you connect securely over non-private networks. The iPhone VPN will work with Wi-Fi (at Starbucks) and cellular networks.
http://support.apple.com/kb/HT1424
http://images.apple.com/iphone/business/docs/iPhone_VPN.pdf