Yeah, the iRiver2 (IMP-250) wasn't sold in the US, because they sold them to RioVolt, and as part of the sale, agreed not to sell their version in the US. Companies hate it when you make it obvious that they didn't actually make their own product by selling the original as a competition to their remarked items
Iriver also just released the IMP-150 in the US, they are calling it the ChromeX now.
The IMP150 and 250 both have the same output stages, i believe. The 350 has an updated one. The 150 and the 250 are both very good players. They just don't have as many cool features as the Slimx. But if you are on a budget, they are probably a very good buy.
As far as hiss, with the amount of discussion and confusion about it, I must remind people not to talk about hiss unless they have actually heard it! There is a lot of jibba jabba about hiss, but not much in the way of actual experiences.