SetCPU or Overclock Widget will serve no o/c purpose, unless you flash a custom kernel onto the phone (Correct me, if I am wrong KSinNY), as the kernel that is in this ROM does not have any clock speeds available over 600 Mhz.
I guess you could slow the phone down, if you wanted, but I am sure you wouldn't want that, except for maybe when the phones screen was off.
I have read somewhere in this forum, that every speed below 600 MHz, actually uses the same amount of juice, so there is no real need to attempt to underclock the phone.