No offense but a million hits/day doesn't have much to do with scalability. That comes down to 11 hits/sec - a single midrange server will deal that without breaking a sweat.
Add one or two orders of magnitude, then you're talking about scalability.
In theory and on average, a million hits per day will come down to 11 hits per second. What you're more likely to see in practice though is huge peaks and troughs in load. The average might be 11 hits per second. But the peak load could be 100 hits per second around lunch and the minimum load could be 2 hits per second at 4 AM.
Moreover, a hit is a rather arbitrary unit of measure. 11 hits per second to the Twitter API is a lot different than 11 hits per second to a static site like example.com.
Yes, I ignored the bell curve due to the nature of the site - should have made that clear.
There's not much point even taking that into account for a read-only affair like SO at such a small scale because the cache is doing the work here and that won't sweat even under 100 or 500 reqs/sec.
Anyways, you point out correctly that discussing scaling-issues is rather fruitless without taking all variables into account. I was just trying to say that "SO (in particular) is handling 1mio reqs/day" does not tell much about the performance of the underlying stack.
Add one or two orders of magnitude, then you're talking about scalability.