RFID Journal posted a story a week ago about a threat to Gen1 tags from a cell phone attack. The attack was detailed at RSA Security's annual conference. It involved a side-channel attack that analyzes the power output of the tag. The power output attack is based off the idea that a tag processing invalid password bits will consume more power.
The story is quite interesting. However, the attack wasn't carried out by a cell phone. Instead the researchers detail a plan that they guess would take about a minute using a cell phone that had been specially modified -- probably through firmware. They also go on to say that Gen2 tags are probably also susceptible although it would take longer, because the Gen2 tags have 32-bits of protection versus 8-bits. (The new RFID credit cards work on the HF band, so they're not a likely target for a cell phone attack.)
Click here for the full article at RFID Journal