More information on this will be available as and when the units start arriving with purchasers and we get a clearer picture of overclocking capabilities and such. To the best of my knowledge the figures from the benchmarking done by the pimoroni.com blog are accurate:
In terms of CPU temperature, the Raspberry Pi 3 runs significantly
hotter than the Pi 2. We used the following to measure peak CPU
temperature while primes were being computed.
while true; do cat /sys/class/thermal/thermal_zone0/temp; sleep 1;
All three Pis idle at 36-38 degrees, and the Pi 3 gets up to a toasty
75 degrees while computing primes on four cores, compared with 52
degrees for the Pi 2 and 46 degrees on the Pi Zeros single core.
It's worth noting that these temperatures won't cause you any
problems. The Raspberry Pi 3 will automatically throttle performance
if the internal temperature of the SoC reaches around 82 degrees
With these figures in mind I'd say you might, if you were planning on throwing a lot of hard number crunching at your Pi, need some kind of cooling to prevent thermal throttling. Under normal conditions I don't think it would be anything to worry about.