I've briefly toyed with a Sprint aircard while in the states this year (not much). It's okay--seems like about 40k or thereabouts. A VPN is usable--web surfing is marginal at best.
As Scott mentioned, Wi-Fi penetration is not yet to the point where one can count on it. I've been stuck in a crappy room at a particular hotel across the way from my office so that I can catch a signal from our office WAPs. Pathetic, but I'm not marching to the nearest Starbucks....
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Saving the world, one clue at a time.