More precisely, there is a hardware cable (cost is about $60 US) from Redpark, which plugs into the 30-pin ipod/iphone/ipad docking connector, and does all the right stuff to provide an RS-232 serial connection. But, to make practical use of it, you also need the GetConsole iPad application. And it all works without jailbreaking your device!
The application should work on the iPhone as well, but I don't have a cable yet to play with the application. I expect to get them both next month, but I have been tracking the progress getting the cable to the market for a few months!
The Redpark folks have a long history of development for Apple systems, and their legacy includes the Keyspan RS-232 serial adapters, another favorite of mine from many years ago.
The GetConsole application is part of a larger project, though, which may become a great field service tool! I'm a big fan of secure remote access to serial consoles (especially the Conserver console server management application). While the GetConsole application will let you attach your IOS device to the serial console of some other device for local interaction, the application also has the ability to be accessed via the GetConsole website. In this way, a field engineer could use his phone to connect to a serial console, but a senior engineer could then use that the phone network to reach through the phone to interact with the serial port.
I look forward to the chance to play with the cable and the application. I'll likely get the Cisco RJ45 version, since I have a large collection of adapters for them (or you can build your own with these adapter schematics). I'm curious if the application will work on my V2 iPhone (which is limited to running IOS 3.x), or if the application requires IOS 4.x. Either way, I remain excited, and I'll post an update in a month or so.