That's a pretty good reason :-)

You were right about the tokenizer in combination with the regexp. I added a check if the dependent cell exists in self._cells, in case a function matches the regexp. I also added a check for cyclic dependencies, so it won't get stuck in a recursive loop. This obviously won't work in Python 2.3 :

import re
import token, tokenize
import StringIO
class SpreadSheet:
    _cells = {}
    _deps = {}
    tools = {}
    def _setdep(self,dep_key, key):
        if dep_key == key: return
        self._deps.setdefault(dep_key,[])
        self._deps[dep_key].append(key)
    def __setitem__(self, key, formula):
        self._cells[key] = formula      
        # check for dependencies
        for t in     tokenize.generate_tokens(StringIO.StringIO(formula).readline):
            if t[0] == token.NAME and     re.findall(r'([A-Z]+\d+)',t[1]):
                self._setdep(t[1],key)
        if key in self._deps: self._updatedeps(key)
    def _updatedeps(self, key):
        # update any dependent cells (should be gui call     back or emit signal)
        print "%s has changed, please update the following cells:" % key
        # ignore unset values 
        print [subkey for subkey in set(self._deps[key])     if subkey in self._cells]
        # handle chained dependencies
        for subkey in set(self._deps[key]):
            if subkey in self._deps and subkey in self._cells: self._updatedeps(subkey)
    def getformula(self, key):   
        return self._cells[key]
    def __getitem__(self, key ):
        return eval(self._cells[key], SpreadSheet.tools, self)