The BeOS schedules threads onto the next available CPU based on a priority schedule. See a recent Be Newsletter article for more details:
If only one CPU is available, all of the threads are scheduled onto that CPU. The BeOS does indeed remain multithreaded, and highly responsive.
In general, we find that single-CPU systems perform very well under the BeOS. Especially now that most computers are powered by chips much more powerful than those used in the original BeBox, we're finding that even a single modern CPU system can actually give better performance than the BeBox 133.