The AtMega8 maximum is 16MHz. This speed has been mainatained so that all the different boards are compatible
Its not the boards that are compatible, products like the Lilypad and Pro-Mini designed to run on 3.3v use the 8MHz clock speed instead of the 16MHz clocks used by the Duemilanove.
The bootloader and runtime software are designed adjust for the difference in the hardware clocks. But some software (like NewSoftSerial) is specifically tuned to 8MHz and 16MHz and although it will recognise which frequency is used by the currently selected board, it will not run correctly at clock rates.