PC3-10600 RAM runs at a clock speed of 1333Mhz. The older PC3-8500 runs at a speed of 1066Mhz. As long as there aren't any other physical or voltage requirement differences between the two types of memory (and there shouldn't be in your case, since both are DDR3 SO-DIMMS), your system should accept it.
Keep in mind that the system's memory bus will automatically adjust to suit the rated speed of the slowest memory module installed into it. So rather than running the memory at 1333Mhz, it will drop down to 1066Mhz and you'll lose some memory bandwidth.
If you want, you can look at this Wikipedia entry to start figuring out exactly how much. As for real world performance, it really depends on what you're doing. Increasing the amount of memory available while losing some bandwidth might be a good idea if you leave everything open all the time.
But if you have specific applications that require a high amount of memory I/O, like a big database for example, speed might be more important. For the average Mac user though, I'd say sheer volume would be preferential.