If Marco says it's true, then I guess it must be true. More of his reasoning is fleshed out here but this quote sums it up pretty good:
Even though it’s (retina display users) a small market today (although don’t forget about the iPad 3), it’s inevitably going to increase substantially in the near future. Don’t you want to get ahead of that? Do you want your site to be ready the first time someone views it on a Retina screen, or are you OK with it looking like garbage for a few years until you happen to buy high-DPI hardware?
I'm not actually going to upgrade any time soon, but my next major computer purchase will definitely have some sort of retina/high-DPI capability.