It should be pretty easy to write a pbasic interpreter for arduino. The mega168 has a lot more program memory that the PIC that implements the interpreter. (OTOH, it should be equally easy to put an entire BS1 into one of the newer PIC chips like the 16F88, and no one has done that either. Perhaps lawyers are involved.)
Similarly, one of my "back burner" projects is to implement a portable "Basic-52" interpreter. If an 8052 could implement a powerful floating point basic in 8k of 8052 assembler and as little as 1.25k of RAM, it should certainly be possible to write a similar interpreter in C that would run on any number of the more-capable micros that are around now with 32k+ of flash and 2k+ of RAM...
(OTOH, where's the arduino library containing near-exact equivalents of the stamp's PBASIC statements/functions? That'd be an obvious first step, right?)