Since coming out, the Samsung Nexus S has had issues that have yet to be fixed or even addressed. One of the issues is that data signal falls off completely and the only remedy is to reboot the phone. It is unable to reconnect to the network no matter how long you wait, it could last all day without a data signal if you are not paying attention.
Another issue is the random reboot of the phone during a call. Both issues have received hundreds of comments on their respective issue number on Google's Issues page with the same story. Google has said that an update is coming soon via OTA, but let's hope that soon means before another Android version comes out.