To acheive the same effect today is actually a lot more difficult than it was over 2000 years ago due to effects of 'Global Dimming'. Although the same experiment was repeated by a Greek solar engineer, loannis Sakkas, along with the Greek Navy who were able to achieve what the Myth Busters team and MIT engineering students failed to acheive. Apperently they had trouble focusing the solar radiation using their copper mirrors to begin with. Perhaps Archimedes may have even design a star cut out to help with aiming.

http://www.time.com/time/magazine/article/0,9171,908175,00.html

Here is a modern scaled up version of this solar device burning through a steel plate.

http://www.youtube.com/watch?v=8tt7RG3UR4c&feature=related