Shellshock, being a bash bug (and affecting systems that use bash for shell commands, etc), is OS and/or application level, and the responsibility of the individual customers to handle (as they are responsible for administration of the instances they run). The Xen bug affects how the hardware is virtualized, and this Amazon's responsibility.
they said it was for Xen before last weekend, no? (didnt read the article, assume they said it was for xen)