In my earlier post on the topic I said "Normally this happens once one is actually elected. Not before one is even officially the nominee." BZZZZZ. Wrong.
What is true, is that in the cases where the transition hasn't started until after the election, they have always been rushed and the start of the new administration has been rocky. A variety of presidents, including Reagan and W himself have started transitions very early. Clinton and Carter apparently started to, but then changed their minds, and their transitions were a bit rocky.
More here, from a guy that tries to turn the story around by criticizing McCain for NOT having started his transition already:
McCain's Presidential Transition Gaffe
(Paul C. Light, Huffington Post, 25 Jul 2008)
The question is not why Obama made the decision, but why Sen. John McCain has not. Instead attacking the Obama campaign for "dancing in the end zone," McCain should have appointed his own planning team long ago.
Obama has plenty of historical precedent to draw upon. On the Republican side of the aisle, Ronald Reagan began his 1980 planning effort in early spring under a senior confidant. The planning produced the fastest transition to governing in modern history, which translated directly into Reagan's victories on budget and tax cuts only six months into the term.
George W. Bush also began his planning early, which produced a remarkably disciplined transition that laid set the stage for another round of tax cuts. It is hard to imagine how the transition could have succeeded without it. Given the Florida impasse, it is hard to imagine how the Bush transition could have succeeded without the pre-election planning.
On the Democratic side, Jimmy Carter and Bill Clinton also began their planning early, but waffled when it came time to use the plans. Under intense pressure from their campaign staffs who rightly complained about a lack of consultation, both decided to start planning again all over again the morning after the election.