What technique is used for thermistors embedded in an IC, and what should I think about when measuring them?
When you make a "thermal resistor", you generally just run a strip of polysilicon for a distance across the die.  I cannot speak for the 68060; however, when I design semiconductors I have an area that looks like an + in the middle of the IC for power distribution, the H-tree of clock.  I generally just run a poly strip, or M1 with well ties across this same space.  It depends on the process and the rules.  

I've attached a photo of an IC that I did that was 6mm x 4mm.  There's a resistor that runs across on M1 that was 1K that should change 5ohm/degC.  One thing to mention is that this 1K is nominal, like your 780ohm number.  The resistance can vary a bit with etching between runs.



If you look around on the web, MOSIS used to publish test results that give you ohm/square for M1, poly, etc.  You can use that to determine what would be best for a specific application.