Kotaku.com: Rumored Xbox 360 Spring Update List
Microsoft has stated that there will be no Spring Update this year. BUT they are releasing a tool that will enable you to attach content you downloaded to any Xbox 360 (only one at a time). This is for people who bought something on Xbox Live Marketplace and their Xbox 360 got the three red lights of death, and got a different Xbox 360 back from repair.
This is currently a (busted) RUMOUR. Don't hold your breath.
Kotaku.com: Rumored Xbox 360 Spring Update Touts "Motion Sensor" Features, Game Installs
I personally don't really care about the motion sensing thing; the Wii sucks ass, and I already have Guitar Hero. I don't need another peripheral.
There is a list of all of the supposed new features on the Kotaku page; check that out for an in-depth look.