I think it's simply because retailers didn't want to take on any more 360 stock when they had those stacks of Arcades and Halo Editions. The retailers figured that if they didn't carry anymore Premiums or Elites, potential customers would settle for the ubiquitous Arcade or Halo Editions and thereby fix their stock issues.
There could be a chance that MS is planning on announcing something at GDC, but I think it's more likely that retailers are trying to clear their stacks of Arcades and Halos.
My Top 5:
Shadow of the Colossus, Metal Gear Solid 3, Shenmue, Skies of Arcadia, Chrono Trigger
My 2 nex-gen systems: PS3 
and Wii 
Prediction Aug '08: We see the PSP2 released fall '09. Graphically, it's basically the same as the current system. UMD drive ditched and replaced by 4-8gb on board flash memory. Other upgrades: 2nd analog nub, touchscreen, blutooth, motion sensor. Design: Flip-style or slider. Size: Think Iphone. Cost: $199. Will be profitable on day 1.
