I had a look at Heljan's website tonight. There's no mention of the traverser part number in the "coming soon" list. Given that the turntables listed as 2Q08 are down for November 08, it looks like they're six months behind where they expected to be at the start of the year. On that basis I would say expect the traverser in late Spring / early Summer next year. I'll still keep a look out for a prototype.
I have also done some rough calculations on how big the 21.5 meter turntables will be. Since they're HO scale (1/87) this works out at about 247mm or about 9.5 inches. Great news for people looking for something for secondary lines and shortish locos but too small for most British pacifics I know of.
So it looks like I have to get the US style one after all.
David