AT&T now has the Lumia 900 back in stock on its website, a relatively quick replenishment after the virtual shelves ran dry several days ago (interested parties were still able to find the phone at Amazon and some brick-and-mortar locations, though). What remains unclear is why the phone went out of stock in the first place — sheer popularity is one thing, but it's plausible that AT&T pulled the phone for a few days to make sure that customers were being sold updated models that didn't have data connectivity issues out of the box. Either way, with Nokia offering buyers a $100 bill credit until the 21st if they take the plunge, it's sure to remain a particularly popular model for the next few days.