The very first thing that occurs to me is 'how secure is it?'
The conventional cellular network is fairly secure, with encryption on data communications sufficient to avoid a 'man in the middle' attack, such as the ones often run on public WiFi networks. These attacks involve the Bad Guys posing as the wireless network, getting you to connect through them. The Bad Guys then route your activity through a proxy server that spoofs the secure sockets layer (like https:// ...), so that encrypted connection to your bank is fully readable! (This attack was infamously run against Citi customers back in 2006. It nicely gets around SecureID and other two-factor schemes.)
The macro cells used in the cellular networks use encryption schemes that can't reasonably be cracked in real time, before a secure sockets layer connection can be set up, so an https:// web connection established over the cell network can actually be pretty secure, better than one over an unencrypted or encrypted-but-cracked public WiFi service. The cellular network is also monitored closely by network operations centers.
These femtocells from Sprint and Verizon allow anyone within range to have calls handed off to the cell, unless the cell's operator has disabled it. (ATT femtocells allow only authorized numbers.) Now, if I were a Bad Guy, how long will it take me to get past the security in that Sprint or Verizon femtocell in my posession? What safeguards are in place to prevent me from tapping all your communications?
Cellular data just got much less trustworthy.
Edit: Already hacked and demoed at ShmooCon.