A post on the Gamerscore blog has revealed Xbox Live will be down for "routine maintenance" on Monday 21st April.
The downtime will begin at 2am PST (10am GMT) and end at 10am PST (6pm GMT).
So does this herald the arrival of the spring update? No, according to Gamerscore: "There will be no immediate or noticeable changes to the interface or services for Xbox Live or the Xbox 360." You never know though, maybe they're testing it out...