The documentation for the builtin round(x, n) says:

"Values are rounded to the closest multiple of 10 to the power minus n."

This isn't always true;  for example:

Python 2.6a0 (trunk:59634M, Dec 31 2007, 17:27:56) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> round(562949953421312.5, 1)
562949953421312.62

Here 562949953421312.5 is exactly representable as an IEEE754 double, so the output from the round function should be 
exactly the same as the input.

Assigning this to myself to remind me to try to fix it someday.