Microsoft has made it official, click here for details.
There are a good number of retailers selling the pre-order (the standard set, Best Buy, Office Depot...), and online you can get it at Newegg.com, Amazon and Tiger Direct.
Microsoft has this note about the duration of the pre-order sale: "The offer begins on June 26, 2009 and will continue while supplies last, or until July 11, 2009, whichever comes first."
This is probably the best deal you could expect out of Microsoft. So pre-order while you can.