Does your current version point to JDK6 even though you’ve installed JDK7?

matt$ ls -l /System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK
8 lrwxr-xr-x  1 root  wheel  59 21 Nov 14:43 /System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK -> /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents

The do this:

sudo rm /System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK
sudo ln -s /Library/Java/JavaVirtualMachines/jdk1.7.0_51.jdk/Contents/ /System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK

If it’s not already, set JAVA_HOME accordingly in your ~/.bash_profile:

export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK/Home

Test it worked:

matt$ java -version
java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)