PG is recommending you get the X.com, but that doesn't mean you need to pay $50,000 for X.com on Day 1 of your startup. I see people making this mistake all the time, buying expensive domains before validating their startup in the first place. Think about what Dropbox, Stripe, and AirBnB did! getdropbox.com -> dropbox.com devpayments.com -> stripe.com airbedandbreakfast.com -> airbnb.com
If your idea seems worthless, a squatter might figure this is the time to shed a worthless domain for a few hundred or thousand bucks. If you're at the helm of a runaway multi-million-dollar success, I imagine the squatter is going to try and squeeze you for significantly more.