Actually, the ASA platform will support NetFlows. Unfortunately, it is only available on the top end 5580 appliance. Here is a quote from a Cisco Rep..

Netflow logging is a high speed alternative to syslog. The 5580 will make good use of its features due to its performance levels. The development team will be watching customer reaction to this feature to determine what the next steps should be. Let your Cisco contacts know you interest.