As someone who hasn't written such a scalable app, but have been interested in the issues around it, this was more or less my first thought too.
App part is usually just "throw hardware at it", but DB part can be really hairy. You can get far on a one machine DB, but once you need to go past that, you got trouble. Unless you've already taken that into consideration.
Also, it's good to be familiar with the common problems and workarounds for scaling. Caching is one such thing. Often you got several possible ways to do things, and some of them lends themselves better to scaling than others. Often with little or no extra work. Or it's just a small tweak to functionality, hardly noticeable that's needed. But something that will be a royal PITA to change later on.
So my advice: Get familiar with scaling problems and solutions, keep them at the back of your mind when creating the site, but don't go out of your way. Not yet, at least. Maybe take time to make easy "hook" areas in the code where you can insert scalability later (f.x design things so it's easy to move parts of it to RPC later on, and make it easy to later add caching to areas).