I bet it would be no worse considering that the plastic backing of the 3G and 3GS aren't blocking the radiation exactly( if it was, how would you ever get a signal?).
That's the big catch 22. Plastic isn't any better than bare exposure. It's a bit like holding up a piece of cardboard to protect yourself from the impending tsunami.
And keep in mind 'radiation' in this sense is not quite the same as 'radiation' in the nuclear bomb/reactor sense. It's all about the amount, and energy of the particles in question. A reactor spews high energy neutrons, electrons and photons (and the occasional high-energy helium nucleus). These are extremely high energy events, and so exposures can be quite low, but the energy levels we are talking about make up for it.
In the case of radio signals, these are always photons, and are fairly low energy particles. With photons the energy in the photon can be measured by its frequency, and these phones operate up through the 2.4Ghz band. To give you an idea, visible light is in the 300Thz range, and x-rays start at about 300Phz.
So each photon emitted from these phones is carrying a lot less energy. But that's not the whole story. The fact that we hold these devices to our head is the main point of concern. It's not fully clear exactly what sort of effect these levels of exposures have. And there are upper limits in the US and Europe on the sort of exposure a device like this can create, which are usually placed well below what is currently considered dangerous.
Another thing to consider is that the radio is never fully active. It isn't as if it is constantly sending out information all day and night. It has transmit and receive phases, and the % time actively sending is usually referred to the duty cycle if my old EE courses are still there in my brain right. You will see higher duty cycles if you are actively uploading/downloading or using the phone to make a call, but as phones share time slots in towers, I wouldn't expect you to see extremely large duty cycles (Assuming you have 100 phones using a single tower equally, the max duty cycle for any phone is 1% for transmission, but this is probably not an accurate number, but neither would large numbers like 10% or higher). Add that with a 0.75-1W transmission power, and you aren't looking at particularly high exposure numbers.
I'd expect the exposure levels to be similar to the 3GS. Sure the antenna is more exposed, but the transmit power can actually be a bit lower as a result, evening it out. The exposed antenna really helps when receiving data from the tower, access point or GPS satellites, as it strengthens reception (which is passive, not active).