Welcome to the forums, Julian! There are several different ways that I could think of this being done. One way would be to connect the USB to serial adapter from the Basic Stamp to one of the Pi's USB ports. The software on the Pi (I am guessing Python) will communicate over a virtual COM port to the Stamp. This way is pretty easy if you are just controlling the servo motors, but becomes incredibly difficult (getting closer to impossible) if there are other electronics connected to the microcontroller. The second option is to connect the servos directly to the Pi's GPIO header, though depending on your electronics experience there is a chance that you may cause irreversible damage to the Pi. I have some Python code that opens a serial port and communicates with it if you want something to use as a foundation for the first method.
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I think Ill try the COM port method (from PI to the Stamp) - I just want to keep it simple initially
Do you have to run and program on the STAMP to make this work ? - or does it run in a "command line" mode?
Julian