The socketed ROM has nothing to do with the processor. The ROM is the read only memory where all the operating system information is stored.
Since the MessagePad 120 was introduced, the ROM has been socketed so that future Newton Operating System Updates can be easily performed.
To date the processor in the Apple MessagePads has not been socketed for easy removal or replacement. The same 20MHz ARM 610 RISC processor used in the original MessagePad is still being used in the current Apple MessagePad 130 devices.