cwlwatcher
Shannon is a completely different market to Cardiff. The vast majority of transatlantic passengers are US tourists who have roots in Ireland. If you look at Shannon's transatlantic destination list, they're all in the heavily Irish-populated North East. Wales simply doesn't have as strong a link with the US, and would struggle to sustain a transatlantic link.
Larger UK airports have a) lost or b) never had transatlantic services, so I doubt that airlines will be looking at Cardiff, which is a much smaller market. I do think that eventually Cardiff will see a Norwegian-style service, or perhaps indirect links via Keflavik.