Clearly nothing scales as well as static files served by a fast web server and any CMS that has to figure out what to load and then load it will not perform as well, WordPress or otherwise. One of the issues is the number of database queries required per URL request and my 2 prior years experience working exclusively with Drupal and now 2+ years with WordPress is that WordPress is much better in that department.
That said, almost nothing with any power is going to scale "out-of-the-box"; it's all about what can you do as your scalability needs grow?
On the low end of "lots of traffic" there are great caching plugins and integrations with inexpensive CDNs you can do a pretty good job on a no-IT budget and low hosting budget. Here are some other questions & answers to review:
There are options for profiling to identify performance bottlenecks:
Once bottlenecks are identified you can do localized optimization with things like the Transients API. This Q&A gives an example that can be optimized using Transients API and shows how:
If you thing really get want to pull out the big guns you can configure Memcached, HyperDB, Nginx and/or more to speed things up (it seems the latter is really evolving into the way to get amazing scalability out of WordPress):
And finally there are emerging WordPress-focused webhosts specializing in performance such as WP Engine, ZippyKid and others:
So the good news is all of the scales very nicely; from the very low end of free and easy with technical complexity and cost only grow as traffic significantly grows. Start small with WordPress and it will be great. If your traffic does grow and you are monetizing it even reasonably well you'll find it very cost effect to scale as you need it.
At least IMO. :)