stab at making this work:

  >>> fromZ3Interface(x) is fromZ3Interface(x)
  True

lib/python/Interface/bridge.py

@@ -14,7 +14,6 @@
 
 $Id$
 """
-
 from Interface._InterfaceClass import Interface as Z2_InterfaceClass
 from Interface import Interface as Z2_Interface
 from Interface import Attribute as Z2_Attribute
@@ -25,9 +24,9 @@ from zope.interface.interface import Interface as Z3_Interface
 from zope.interface.interface import Attribute as Z3_Attribute
 from zope.interface.interface import Method as Z3_Method
 
+_bridges = {Z3_interface: Z2_Interface}
 
 def fromZ3Interface(z3i):
-
     """ Return a Zope 2 interface corresponding to 'z3i'.
 
     o 'z3i' must be a Zope 3 interface.
@@ -35,36 +34,35 @@ def fromZ3Interface(z3i):
     if not isinstance(z3i, Z3_InterfaceClass):
         raise ValueError, 'Not a Zope 3 interface!'
 
-    if z3i is Z3_Interface:  # special case; root in new hierarchy!
-        return Z2_Interface
+    if z3i in _bridges:
+        return _bridges[z3i]
 
     name = z3i.getName()
-
-    bases = [ fromZ3Interface(x) for x in z3i.getBases() ]
-
+    bases = [fromZ3Interface(x) for x in z3i.getBases()]
     attrs = {}
 
     for k, v in z3i.namesAndDescriptions():
-
         if isinstance(v, Z3_Method):
             v = fromZ3Method(v)
 
         elif isinstance(v, Z3_Attribute):
             v = fromZ3Attribute(v)
 
+        #TODO bridge Fields to Attributes?
+
         attrs[k] = v
 
     # XXX: Note that we pass the original interface's __module__;
     #      we may live to regret that.
-    return Z2_InterfaceClass(name=name,
-                             bases=bases,
-                             attrs=attrs,
-                             __doc__=z3i.getDoc(),
-                             __module__=z3i.__module__,
-                            )
+    z2i = Z2_InterfaceClass(name=name,
+                            bases=bases,
+                            attrs=attrs,
+                            __doc__=z3i.getDoc(),
+                            __module__=z3i.__module__)
+    _bridges[z3i] = z2i
+    return z2i
 
 def fromZ3Attribute(z3a):
-
     """ Return a Zope 2 interface attribute corresponding to 'z3a'.
 
     o 'z3a' must be a Zope 3 interface attribute.
@@ -75,7 +73,6 @@ def fromZ3Attribute(z3a):
     return Z2_Attribute(z3a.getName(), z3a.getDoc())
 
 def fromZ3Method(z3m):
-
     """ Return a Zope 2 interface method corresponding to 'z3a'.
 
     o 'z3a' must be a Zope 3 interface method.