I've been to the confluence of the Imnaha and the Snake. It was during pretty high water on a jetboat outing. The other time was after a multi day trip and I completely spaced it. The runout looked pretty fun, but even with the higher water it looked kind of boney. Grant Amaral's (sp?) book has a little write-up on it. I'll check it out to refresh my memory. I'm pretty sure it's listed as III(IV). Pretty sure you could make the creek happen in whatever boat you were using for the multi-day, but again I'll check the river description.
I agree with Yotes that it is often overlooked. Most likely because the time you get there you're A) either tired from paddling all the flatwater B) your group has a raft/cat with a small motor and you're all drinking beer by that point (my preference) or C) you completely space it from either reason A or B.
Anyways, Yotes I'm down for it. I'm sure that there will be a few multi-days down the Salmon/Snake this year. The section we did this past summer was also great for beginners (if you have raft support).