(And it is more about "not a problem" than it is about architecture, because a gigantic proportion of all startups will never have any scaling problem to speak of, and the majority of the remainder will be able to solve theirs in fairly boring fashions.)
Almost all startups would be better served by playing around in their technology of choice looking for something that could possibly cause a scaling problem rather than planning for scaling problems.
Development speed is way more important than scalability. Like Paul Graham says, the vast majority of startups fail because they make a product no one wants or because they never launch a product in the first place. Faster software development means you figure out if your product is a dud faster, and you're less likely to run out of morale before you've got something substantial.
I've been working on my current (unreleased) project with Django and jQuery on Linux for about a month. Here's all of the stuff I've made use of from the Django, jQuery, and Python communities:
http://code.google.com/p/django-command-extensions/
http://github.com/dcramer/django-ratings
http://github.com/pydanny/django-uni-form
http://github.com/robhudson/django-debug-toolbar
http://github.com/alex/django-ajax-validation
http://orkans-tmp.22web.net/star_rating/
http://jquery.malsup.com/form/
http://plugins.jquery.com/project/ScrollTo
http://jashkenas.github.com/coffee-script/
http://code.google.com/p/modwsgi/
There's no way I could ever use a niche framework after having drunk this reusable code kool-aid.
Even if Django weren't scalable, if I really was on the road to being the next Facebook or Youtube, there'd be plenty of time and money to rewrite in something that could scale.
yeah - friendster.
Twitter would be an example of a startup that experienced severe scaling issues and still ended up on top.
Know any others?
2. prefer static content over dynamic, as much as possible
3. lots of machines, with the requests distributed across all of them
these 3 strategies alone will buy lots of scale
Our sites run on a standard LAMP stack (load balanced across between 2 and 25 machines using an F5), memcached for object and query caching, caching at the application level, varnish and nginx for file caching, and limelight CDN for serving images, css, and javascript.
I don't think that the technology stack matters that much, though.
I would also say it's a massive and on-going trial and error process to see what actually works, the biggest sites are still learning and falling over.
Haven't reached 1M hits per day yet, but I'm about to experience a bit of preemptive load testing to find out where the initial problem spots may be.