In previous years the rhubarb has started it's rebirth on April 15 (2008) and April 25 (2009). With only a three-year track record, it's hard to say if it's early or average timing, but most likely average since spring 2009 was generally on the late side.
It's located on the north facing side of the garage so somewhat less affected by early Chinooks (shaded soil there taking longer to warm up than soil in the sun). With today's high reaching around +18 C after a cold week, the temperature in combination with daylight hours seems to have convinced it to wake up.