There have been a few recent cases of those generally of Middle Eastern descent being detained or arrested with large numbers of cell phones in their possession. The case of the Georgian illegal alien in Pennsylvania from a few days ago.

And, from Caro, Michigan: Three men - apparently naturalized citizens from Texas via Palestine - bought 80 cell phones from a WalMart. The cops found 1000 other phones in their van. They also had a laptop "with store addresses and store logos".

And, from Marietta, Ohio: Two men from Dearborn, Michigan were stopped with several prepaid phones and have been arrested on an unnamed charge. A week earlier, another Middle Easterner from Dearborn was stopped with a large number of phones. Debbie Schlussel has been covering this story, however from a comment on that thread we get a possible more innocent explanation (something I already figured as a possibility):

What is going on is tracfones are sold in the US at a substantial subsidy and low minute intial purchase phones are way cheaper than buying phones outright.

Tracfone works on GSM and it is easy to remove the chips, unlock the phones subisidy lock (not in itself illegal) and ship the phones for a good profit. Indeed a HUGE number of new unlocked tracphones are sold in the US on EBAY everyday for about $20 over retail. bulk buying in lots of 10 to 20k is at $25 a unit...

Looking at walmart this week they had a tracfone on sale for an unusually low $15wiht calling credits -- and this phone is currently on sale on ebay for $60 new unlocked...

While those in the last incident also had more suspicious items such as flight and passenger information, and there are links between them and an airline worker. So, the purchases might have been for detonators, or they might have been designed to test our defenses or make us worried, or their boss might have just trying to make some money on the gray market.