I know the reddit guys, and I can tell you it's definitely not that. Hitting the DB is extremely rare - almost everything comes straight out of cache (either memcache, or memcachedb for caches which are more expensive to rebuild). Execution cost is dominated by the python app servers.
The overwhelming majority of reddit's pageviews are users that are not logged in, which is very very cacheable.
I am pretty sure this is false. While the overwhelming number of unique visitors to reddit are not logged in, those users are far less engaged than average. Logged in users make up a huge percentage of the pageviews because of their higher engagement level.
About 20% of the users are logged in, and they represent about 50% of the page views.
The database is rarely accessed in request. Almost all DB access comes from batch processes.
Yep. That's why you shouldn't have pylons noobs running a site like reddit.