I was poking around with "permitted IP addresses" for Callcentric just now and when I looked at my dashboard, I discovered they now allow "Permitted addresses" to be entered in CIDR notation! Don't know when they added the feature, but it could be a boon for DSL users who must use a dynamic IP.
*Edited*
I had to look it up, too. CallWithUs recently implemented this feature. That was why I began poking around. Now if we can just get Obi to implement the "Only talk to the IP we're registered to" feature, we should be able to lock it down pretty well.