Just like Japan, in the US there'll be a 20MB download limit attached to the 3G version of the PlayStation Vita, Sony announced yesterday.
The limit of course only applies to data being downloaded over a 3G network; if you're downloading via wi-fi, on either the 3G or wi-fi-only models of the Vita, you can grab bits and bytes to your heart's content.
Interestingly, Sony explains that the limit has nothing to do with the hardware of the Vita. It's instead a limitation on AT&T's part, the Vita's exclusive mobile carrier not allowing 3G downloads any bigger than 20MB on any handheld devices on its network.
PS Vita 3G 20MB download cap confirmed [GameSpot]