Some people here are clueless. I see lots of suggestions of a useless $50 price drop just before a GTA4 launch.
Forget that!
What did we (and MS) learn from last fall and holiday sales? That game library matters a lot, but price matters even more. Sony gained LOTS of ground by going to $400. MS were confident and only reacted by adding 2 games as packins during christmas season (which btw practically was to drop the Premium to an effective price of $300). Effect: MS only managed to sell "only" 1.25 million consoles in Dec, not the 1.5 mill even Pachter had predicted.
Next price drop will be $100 (or if they really like an odd number, it will be $279 for the Premium and thus only $80 off).
Time? Well probem is, GTA4 has not only been delayed, there's actually no clue about a release date at all! It could well be in june or even july. No way MS will wait that long.
By tradition price drops can happen in march. But if by late february, when it's time to announce the forthcoming drop, a more exact release date of GTA4 is known, MS might strategically split the 100$ drop in two: 50$ in march and another $50 a week before GTA4 release.
Mark my words: 100$* within the first ½ of the year. X360 Premium for $250. 
(and all those naive people that today are so very impressed by the increased PS3 sales and predicting Sony will come 2nd this gen after all, they will all go: oops... damn, we're last again, Sony lost this gen) 
*The Arcade SKU would obviously only drop $80
