High-Availability for site-to-site VPN using HSRP

http://sites.google.com/site/amitsciscozone/home/ipsec/ha-for-site-to-site-vpn-using-hsr

http://www.cisco.com/en/US/partner/prod/collateral/iosswrel/ps6537/ps6586/ps6635/white_paper_c11_472859.html