I think the US could gradually move to metric, but as with the UK, there's no need to replace miles as a distance measurement. In some ways, I think it'd be nice to always keep mph/miles... but otherwise, for weights and measures, metric would be superior.

And hey, if it's good enough for NASA, it's good enough for the rest of the states. ;)

Worst hit, would probable be the automobile industry. But I'd say there'd be advantages to metrification there too.