How can I make a dictionary that marks itself when it's modified?

[sandravandale at yahoo.com]

It's important that I can read the contents of the dict without
flagging it as modified, but I want it to set the flag the moment I add
a new element or alter an existing one (the values in the dict are
mutable), this is what makes it difficult. Because the values are
mutable I don't think you can tell the difference between a read and a
write without making some sort of wrapper around them.

Still, I'd love to hear how you guys would do it.

Thanks,
-Sandra