Thanks Dan, but my problem is that it's a large extent area that would require high resolution cells in order to emulates the smoothness of the original IDW... I could chop the data into sections and process it, I guess. I'm probably being to fussy with the convert, to be fair.
But basically, the only way to do this is to export as a grid and then do the raster to polygon with smoothing activated.
It's a shame really, as the IDW visual is vector-like and it's a shame you can't grab that 'visual' and translate it into a real data layer.