The European digital music service Spotify finally launched today in the U.S., and reviewers seem pretty impressed. But while Spotify has a mobile client for smarthpones — as do its major competitors — I’m still skeptical about the market for on-the-go music. Don’t get me wrong: demand for mobile music is as strong as ever. But as my colleague Kevin C. Tofel recently documented, listening to decent-sounding music over cellular networks eats into data plans pretty quickly, which is a huge problem in the era of capped data plans. And while Wi-Fi connectivity is becoming ubiquitous, it’s the cellular support that makes such services truly mobile. Pandora has proven that people want on-the-go music, but I don’t think many will be willing to pay heft data charges in addition to those subscription fees.