Yes, WordPress 3.0 uses less CPU and memory than 2.9.2, but all of this depends on your definition of "high CPU usage." Most issue with high CPU usage are usually caused by one of three things:
1. Poorly coded plugins. Try decativating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.
2. Ridiculously low CPU caps. Some hosting providers, particularly low-cost providers, cap your maximum CPU at a ridiculously low level. This can give the illusion that WP is prone to "high CPU usage," when in fact the CPU usage is actually low to moderate.
3. A very high traffic blog with lots of plugins. If this is the case, consider caching.