The Western church (as it relates to Protestant Christianity) has been dying for some time now. However, I wouldn't really use the West as any sort of thermometer for Christianity's influence. The underground church in oppressive countries has been growing at a fairly steady rate for some time now. From my perspective, I'm seeing more of a shift than anything else. I suspect it won't be long before countries start sending Christian missionaries to the US (I know of at least a few cases of this already happening).

Source: I work within a para-church organization that keeps track of this stuff.