It depends on the content. If it's something that will be visited just by HN/Reddit users: Sure, you're not going to get 2k/sec directly from them. But if it's something that's going to get shared/tweeted/retweeted/blogged/reblogged/etc., then you will be looking at huge traffic numbers. It just depends on how viral your content is and how far it spreads. It's the long tail that matters.
You've really got to hit the jackpot to get up to 2000/request per second (assuming you don't already have a high traffic site). Assuming about 20 requests per pageview, that's more than a quarter million pageviews per hour.
I've had things go moderately viral (thousands of tweets / retweets) and you don't get anywhere near that. Pre-tuning for hitting the jackpot is in most cases going to be premature optimization.
Spikes of 2000/second are not unrealistic, and part of the problem is that once the spike hits, inadequately designed applications start to wedge on lock contentions and just plain inadequate CPU, the VM layer starts swapping, people trying to get to your site start hitting reload (humans do not have an exponential back-off function), and the whole thing comes crashing down until traffic drops off to more sustainable levels.
