Can Drupal scale to 100 users?

I was talking about SCF and Drupal yesterday with someone I'm working with on another project, and he said he finds Drupal difficult to scale. He thinks that it's not capable of handling more than 100 users. If that's true, that would be a major concern for [our project]. What has been your experience regarding scaling issues with Drupal?

My first response to that was in internet-speak: LOL.

Then I recovered my professional self:

I'm assuming your colleague is referring to logged-in users, since there are simple tools to make Drupal scale for anonymous visitors (not logged in) just as well as a static HTML site (not that we tend to put these tools in place, because Drupal does run efficiently enough, even with lots of extra modules and functionality, to serve fully dynamic pages to many users at once, on reasonable hardware of course).

There are, of course, plenty of examples of Drupal sites that handle many times that number of users, even concurrently:

http://drupal.org itself has well more than a quarter-million users
http://www.nowpublic.com/ has several hundred thousand users

And these sites are Drupal and all see lots of users and activity:
http://kerneltrap.org/
http://teamsugar.com/
http://ourmedia.org/
http://www.standagainstpoverty.org/
http://www.incbiznet.com/
http://www.observer.com/

A couple, kernaltrap, stand against poverty, ourmedia, and to a lesser extent even drupal.org (which has had uptime issues on occasion) probably have a similar level of financial resources to Alzforum.

Agaric's own experience with scaling with Drupal is very positive, with the exception of multilingual sites where various small bugs in modules regarding use of Drupal's translation functions can cause significant performance problems.

Resolution