CoolThreads is a multi-core design that currently scales up to eight cores on a single die. Given that each core is capable of executing up to four threads simultaneously, this means a single chip can execute up to 32 threads.
CoolThreads chips will be available at clock speeds between 1.0 and 1.2GHz, and should consume no more than 70W of power. It will be interesting to see how Sun provides enough bandwidth across all eight cores to keep data from bottlenecking; there's only so much room for cache in an eight-core design.
Those interested in more in-depth information on Niagara can find it here, at Ace's Hardware. This write-up might be somewhat dated, but very few, if any, of the fundamentals it discusses appear to have changed.