I think the answer lies in between a few statements by various sites and business-oriented sites.
1) The move isn't a move of desperation from Microsoft. The Xbox 360, for the Premium bundle, has come down a mere $50 ($399>$349) since it debuted, and the Arcade has come down $20, but included more items in the bundle. Microsoft has been far more aggressive in Europe (where it slashed prices by a similar amount in February), and Japan (which dropped it's price quite some time ago).
2) Microsoft will not lose money on dropping the price. The console is hitting the 3 year mark very soon in terms of production. You would have to be very dense to think that Microsoft hasn't lowered the cost of production from $513 per Premium to lower than $299 in 3 years, and similar margins on the Arcade. As I've stated before, when we broke the price cut story, Microsoft is most likely making Arcades for $143-179 per unit + shipping and misc. fees.
3) Microsoft will not phase out the Arcade with it's larger-than-the-rest price cut on the Arcade. They have been stating for the past year that they know the sweet spot starts at $199 for a console...I think Shane Kim was quoted as saying "70% of a console's sales are at, or below the $199 price point, and we know that". Microsoft needs a product at $199 to boost sales in it's casual/core division...And it's very unlikely that the Pro/Premium will sell at that price for another year or two.
4) As for the effect on the market. That's a tough one to call. In Europe, a similarly-styled price drop has boosted the X360's sales to 100% YOY versus last year - which proved the aggressive price drop worked for Microsoft. I think in the United States, Microsoft will see major gains against the PS3 and Wii. I don't think it'll actually hurt either console, but this move will lead the way to see Microsoft have it's biggest year yet, beating it's 3.2 million cume from September to December in North America, which it sold last year.