After updating to iPhone 3.0.1 (the SMS security fix) Xcode refuses to build and deploy to the iPhone, claiming it doesn’t support that version. However, 3.0 (and not 3.0.1) is currently the latest supported version.
To fix this, execute the following (everything in one line) in a terminal to create a softlink for 3.0.1 to 3.0:
ln -s /Developer/Platforms/iPhoneOS.platform/DeviceSupport/3.0\ \(7A341\) /Developer/Platforms/iPhoneOS.platform/DeviceSupport/3.0.1
For more information, see the iPhone OS 3.0.1 Advisory available at the bottom of the iPhone SDK page (in the downloads section).