Hi Alex,
Here's one for CME:
http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/srnd/design/guide/security.html
This is a good one to prevent toll fraud:
http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_tech_note09186a00809dc487.shtml
You're more likely to have toll fraud by having an open public IP address port 5060 than you are anything else. Of all the security issues I've seen people experience, nearly every one of them has been because of this.
Government and other people with more regulation will sometimes be forced to use SRTP, but it is rarely worth the hassle and added complexity.
Hope this helps.
-nick
