After having installed Netbeans 7.0.1+dfsg1-5ubuntu2 I found that I cannot open old projects. Netbeans will find list the project directories, but will not open the projects. The solution is quite simple.

The problem seems to be that the java version selected by default is not compatible with v7.0.1 of netbeans.

sudo update-alternatives --config java

There are 2 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                           Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-7-openjdk-i386/jre/bin/java   1071      auto mode
  1            /usr/lib/jvm/java-6-openjdk-i386/jre/bin/java   1061      manual mode
  2            /usr/lib/jvm/java-7-openjdk-i386/jre/bin/java   1071      manual mode

Pick option 1 to switch to java v6 and restart netbeans.

Once finished working with netbeans, you probably will want to switch back to java v7 (auto) by repeating the above and selecting option 0.