Do you know iPhone supports the Cisco VPN Client?, yes both the iPhone Software versions 2.x and 3.x supports L2TP, PPTP and IPsec type of remote access VPN connectivity.  The IPSec option is actually Cisco VPN client software for communicating securely with Cisco ASA and  PIX firewall.

According to Cisco only ASA and PIX firewall supports the iPhone Remote Access VPN, where as the Cisco IOS routers and bit older VPN 3000 concentrators will not support the iPhone VPN features.

By using this feature mobile workers can connect remotely to their Enterprise network via secure VPN tunnel using their iPhone. Both the Wi-Fi and Mobile Data Networks can support the iPhone VPN client to set up a tunnel between an iPhone and their Enterprise network. Following authentication methods are supported for establishing the remote VPN tunnel

ü  Password

ü  RSA SecurID

ü  CRYPTOCard

ü  Certificate

For more info on how to configure your Cisco ASA firewall do check this link from Cisco Systems.

http://www.cisco.com/en/US/docs/security/vpn_client/cisco_vpn_client/iPhone/2.0/connectivity/guide/iphone.html