Apparently, the answer is "throw up your hands and turn the URL over to Mobile Safari," as you can see in the screenshot here. I'm not sure this is a particularly well-thought out workaround; there are plenty of other apps that can pull down adult-themed content, starting with Apple's own YouTube tool and including ebook readers, RSS feed handlers, etc.
Should a developer providing a custom portal to an external website really be responsible for all the content hosted there, or would it be simpler to put up the same 18-and-up warning that Craigslist uses? It may be up to Apple to implement, or allow a third-party to build, more granular parental controls for iPhone apps, but simply handing off the same mature content to Safari for display seems like a pretty big cop-out.
Update: In response to some of the commenters, we don't know whether this restriction was put in out of an overabundance of caution, or because of a specific requirement of the App Store -- so blaming Apple for prior restraint may be premature. We'll try to get the straight story.
Update 2: It has been confirmed that the change to the craigsphone behavior was, in fact, prompted by a concern from Apple's app store team that the app might be violating terms of service by displaying the mature content. If other apps have been given similar red flags, please let us know.
Thanks xnifex & Nilay