I have my bed sprayed with a rhino-liner coating and I can say I am really happy with the result.
I don't know about spraying the frame, but it sounds like a great idea. Don't forget that your entire frame will suddenly no longer be a direct connection to ground! That could lead to some whacky electrical troubleshooting when you get to wiring it back up.
Are you planning to spray it yourself or have it professionally coated? I researched both before having my bedliner done and the differences are huge. The professonal coating is much thicker.
I guess in the end its not all that much different from the "undercoatings" that are on vehicles from factory applications.