This feature has been added in Simulink 6.0 (R14). See the following section of the Simulink documentation:
Accessing Block Data During Simulation :: Working with Blocks (Using Simulink):
For example, to access the output data from a particular block during simulation, perform the following steps:
1) Set the simulation stop time to 'Inf'
2) Start the simulation
3) Click on the block you want to access
4) Type the following at the MATLAB Command Line:
rto = get_param(gcb,'RunTimeObject');
If you are using a previous version, see the following:
The ability to do read the value of a signal from the MATLAB command line during a simulation is not available in Simulink.
To work around this issue, you can write a MATLAB file S-Function to send data to the MATLAB workspace during the simulation. For more information, see the following Related Solution "How do I log a signal from my Simulink model to the MATLAB workspace at each simulation time step?"