Can the frequency of the SPI sck on an mbed be adjusted?
Yes you can change the frequency of the clock,
http://mbed.org/projects/libraries/api/mbed/trunk/SPI#SPI.frequency


  frequency
  
  void frequency( int     hz   =  1000000 )
  
  Set the spi bus clock frequency
  Variables
  hz  SCLK frequency in hz (default = 1MHz)