Performance penalty for using properties?

Now that I'm back to Python and all the new (to me) cool features,
I find I'm using properties a lot, i.e. I'm defining:

	foo = property(fset=..., fget=...)

for a number of properties, in many of my classes. I'm not using
them for anything performance critical yet, but could see myself
doing so in the future. Can anyone comment on the performance
costs associated with properties vs. simple attribute lookup?

Thanks,
Ken