Whether this is true or not, Microsoft is pretty smart for suggesting the PS3 will be $399 by Christmas.
We really don't know how many 60GB PS3 systems need to be sold before the $599 SKU is the only PS3 that is available but (for argument's sake) assume they have enough systems to last until September 1st.
The suggestion of a $399 model from a 'respected' source will spawn all kinds of rumors and news stories (and endless blog posts). In september when anyone even Googles "PS3" they will be forced to read this speculation; some people will even misunderstand the articles and assume the manufacturer of the PS3 was announcing a price reduction (either they think Microsoft produces the PS3 or they have a brain fart and misread Microsoft as Sony). The result of this is people begin anticipating a price drop on the PS3 and hold off buying more systems ...
This puts Sony in a bad position, either they reduce the PS3's price again (start looking desperate by reducing the price of the PS3 twice in 1 year) or they lose the sales of people who misunderstood Microsoft's intentions.