There has been a lot of activity lately on the Drupal Consulting mailing list to do with scaling and optimizing Drupal. We often work on sites that need to handle a few hundred thousand visitors a month. I find that there are a few things that must be implemented as the basis of a scalable Drupal foundation:
1) Enable APC caching: This is a much simpler implementation than the MemCache set up, and most developers with intermediate PHP experience can implement it;
2) Enable MemCache: This may be more challenging to implement, but is also found to be more effective than APC;
3) Indexing Table Columns: This will allow the SQL joint queries to run more efficiently;
4) Ensure that SQL queries are optimized: For example, making certain that only the necessary fields are selected instead of using SELECT *.
This is only a basic list of options to optimize Drupal. There is much more to it in terms of server configuration to make a Drupal site scalable. Scaling Drupal is a relatively specialized field, and there are only a handful of firms that I know of that specialize in doing this:
I am sure that there are other firms out there that specialize in Drupal scalability consulting. Please feel free to add your company link in the comments below.