New-style objects are not instances, apparently

[eefacm at gmail.com]

I have a class that derives from Exception.  In Python 2.4,
isinstance(MyClass(), types.InstanceType) was True.  In 2.5, it's
False.

Further experimentation showed that derivation from object was the
culprit; new-style objects are not considered "instances" in the above
sense.  I wasn't able to figure out a workaround.  Is there one, or is
the distinction between traditional classes and built-in types only
going to get more and more hazy?