Thoroughly cooking stuffing in a Turkey
I am firmly in the "stuffing is evil" camp... but lets take that as read :-) 

If you absolutely must have in the bird stuffing, here is a link to a (I hope legal) excerpt of Alton Brown's Good Eats, showing his technique for doing turkey with stuffing:

http://www.aol.com/video/alton-browns-turkey-with-stuffing/444711017/

He uses a food-safe cotton bag, and pre-cooks the stuffing in the microwave.  Then, he uses what looks like a flexible plastic cutting board to guide the bag of stuffing into the turkey.

The summary of his technique is that the stuffing is partially pre-cooked, so that it is hot when it goes into the bird, and comes up to temperature together.

. . .

In the abstract, independent of the pre-cook the stuffing method aluded to above:

The temperature of the inner surface of the cavity is going to be essentially equal to the temp. of the outer surface of the stuffing. Since neither is in contact with any of the heating modalities in the oven (radiation from the hot oven walls, conduction from air contact, convection from circulating hot air), that interface will only heat by conduction through the outer turkey. You want cooked stuffing without overcooking the turkey--that is tricky. But the short answer is yes, the stuffing prevents the turkey from getting convection/conduction heat on the inside.

To assess the temperature at the inside of the turkey, take the temperature of the stuffing, at the edge of the cavity. It will be safe at approximately 155 F (there is some variance depending on your assessment of risk, and which source you look at). This only indicates the turkey is safe, not the center of the stuffing, which you should also check.