Some parts of it are being rewritten from the ground up, and I know of at least one sizeable portion that's being rewritten in python.

edit: depending on who you meant by 'they', I bet go would be a contender for (parts of) a total rewrite

YouTube is starting to use Go quite a lot. They even released this: http://code.google.com/p/vitess/

What does "grandfathered" mean? Seen the term in a few recent HN posts as well, dealing with accounts at web sites where they change plans, etc. TIA.