SMP for games will take off quickly, as more and more gamedevs look at Cell programming, etc. it will become closer and closer to double perf. with dual-core.
At the moment, the main concern about making games multi-threaded(and thus able to use more than one CPU) is that with only 1 CPU, having the second thread immediately causes an ~1% performance loss and a 5-20% increase in code complexity. Also if you launch a second thread, the threads will alternate at (very)approx every 5-20ms, unless they're tightly synchronized, this will cause delays before the code can move between jobs.