Code Golf: Ulam Spiral

Answer 1

MATLAB, 56 characters

based on @gnovice solution, improved by using MATLAB's spiral function :)

disp(char(isprime(flipud(spiral(2*input('')-1)))*10+32))

Test Cases:

>> disp(char(isprime(flipud(spiral(2*input('')-1)))*10+32))
2
* *
  *
*  
>> disp(char(isprime(flipud(spiral(2*input('')-1)))*10+32))
3
*   *
 * * 
*  **
 *   
  *  
>> disp(char(isprime(flipud(spiral(2*input('')-1)))*10+32))
5
    * *  
 *     * 
* *   *  
   * * * 
  *  ** *
 * *     
*   *    
 *   *   
*     *