Semantics are important
So why is Tony Blair still PM rather than having been replaced this week?
The bottom line is that most Labour people want to keep the Party's record of never having forced a leader out of office - every one so far resigned of their own accord or died in office.
This meant that a big lump of the people who may well vote for Brown as leader were happy to demonstrate their loyalty to Blair at the point when it mattered.
The issue is one of semantics - there are a whole mass of people who see Brown as Blair's natural "successor", but not his involuntary "replacement".