I don't know, I'm pretty sure that no American convertible has them (Vette, Sebring, Camaro/Firebird, Solstice, XLR, PT Cruiser (has a fake rollbar), Viper (has hoops that are too short, according to either Motor Trend or Car & Driver))
Also, the Toyota Solara doesn't, nor do the Jag, the MR2, or the Miata.
But all others seem to (TT, Mini Cooper - both fixed hoops, the Beetle, all Porsche, Mercedes, BMW, Saab, Volvo).
I think the Crossfire does, but that's because it's based on a Mercedes, I think.
So, it's not really clear cut, some really expensive cars don't (Vette, XLR, Jag and Aston Martin) and some really inexpensive cars do (Beetle, Cooper).
I know it's a rare type of accident, especially with a low slung car, but I've seen it happen, and when it does that angled windshield frame won't help much.
The new Mustang is such a safe, heavy car. My guess is, in the 2007 or 2008 model year we'll see some sort of pop-up system behind the rear seats, or at least I hope so...