I've been hit by a bit of news from a friend that first scared me: updates to the DnD 4th Edition core set of books.
It immediately brought back memory of the painful "3.5" edition books that came up not so long after I had purchased my "3.0" books.
I took a quick look at the changes in the Player's Handbook (I don't own / am not familiar with the others) and found that there were all simple changes, and fixes. In fact, I took note of them on post-it notes which I spread all around my 4th ed PHB.
Not much to report on the changes; they all are details about rules, adjustments in wording - even an omission as to which attack vs defense stats to use with a power. None of these changes would warrant a "4.5" edition book - though they could (and should) be fixed in ulterior prints. Who knows, maybe they will print a 4.5 edition with these changes. But so far, there wouldn't be any need to purchase them if you have the ones that are out now 
You can fin the changes in here: http://www.wizards.com/default.asp?x=dnd/updates