I have an ASA 5520 set up to receive VPN connections. I don't have any issues when the remote end is a ASA 5505, but I notice when the remote end is an 1841 that the VPN connection drops after an extended period of time. I am guessing that this issue will be the same for any IOS router. I have been working with TAC and still have not solved anything. We were sure it had something to do with the SA lifetime and we set both ends to have the same ISAKMP and IPSEC lifetime, but that didn't make a difference.
I am sure I am not the only person using an ASA with a 1841 remote router. Just for troubleshooting purposes I have an ASA 5505 on the same DSL circuit as the 1841, both configured with Public static IP addresses and the ASA 5505 never loses connectivity with the ASA 5520. So its not the dsl circuit.