I hadn't heard about the Yahoo/PHP thing. When did that happen? And from what I've read, Google is still mostly C++ and Java, with Python a distant third: smart humans (think Norvig) have an easier time thinking in Python, but computers (see Norvig: http://norvig.com/python-lisp.html ) aren't so lucky.
No company the size of Yahoo or Google uses just one language for everything. Google has Guido on their payroll and Yahoo has Lerdorf on theirs. It's a cultural choice more than anything.
