I found the following quote by Microsoft's Ray Ozzie, in The Web 2.0 World According to Ozzie, to be fascinating:
"In terms of managing trust boundaries, one of the huge challenges that enterprises are going to have is...managing trust between components of composite applications...
"We believe there should be significant auditing within service components—such that when you do expose a partner to certain enterprise data...you have a complete record of the kinds of things that their app did." (emphasis added)
I think Mr. Ozzie is advocating application security monitoring, a cousin of network security monitoring. If Mr. Ozzie is being as clever as I think he might be, he's realizing that it's going to be nearly impossible to run Web services and the like "securely." We're going to have to rely on monitoring and response since prevention will be far too complex. Resistance will be tried, but will be -- you guessed -- futile.