If you don't want to get involved with locales here is a function that formats numbers:
def int_format(value, decimal_points=3, seperator=u'.'):
value = str(value)
if len(value) <= decimal_points:
return value
# say here we have value = '12345' and the default params above
parts = []
while value:
parts.append(value[-decimal_points:])
value = value[:-decimal_points]
# now we should have parts = ['345', '12']
parts.reverse()
# and the return value should be u'12.345'
return seperator.join(parts)
Creating a custom template filter from this function is trivial.