Are you coding your websites for massive scalability?

I do, and I've done so. Never alone though, always as part of a team.

Things like load-balancing, reverse-proxies, database partitioning/clustering/replication/etc, SAN, etc?

Yes. And except for reverse-proxies, I've done all the things you mention for non-Web related services as well. Wait, if you consider MTUs relaying mail to another internal server as a reverse proxy (which it is, although not for HTTP traffic), I've done reverse proxies as well.

If so, what web development framework are you using?

mod_perl, HTML::Mason, Tomcat, Websphere, Silversomething and some HP technology whose name I cannot remember.

Have you *actually* scaled your project out across more than one server?

Yes. Scaling web services over more than one physical server is trivial; for more interesting is to scale your database server over more than one.

Have you *actually* written unit tests for the web portion of your application?

Yes.

Did you use WWW::Mechanize or something else?

Yes, and yes. (X-runner, wget, nagios, lynx -src, LWP, C, IO::Socket::INET, ...)

The reason I ask is that aside from the daily barrage of 101-level questions, I don't see much in the way of these real-world problems coming up here.

I'd say most of the 'real-world' problems aren't Perl problems. I certainly wouldn't post non-Perl problems here.