Yes it does happen when I use -d 4.  Just confirmed.

If you'd like remote access to this box I'd be glad to give it to you - you can mine wherever you want with it, develop whatever you want.  It's basically my cudaminer playground right now, and I assume it would be nice for you too.  Shoot me a PM and we can gtalk/IRC/whatever you prefer if you're on the Telegram wagon.

When I use -d 4 I'm getting 96w idle with 6 cards spinning fans, but the 6th card never spins.  In terms of hardware I am using the same USB risers that I was using with 6 x 280X on the same motherboard (I had them undervolted and 2 PSUs).

For people asking questions about risers... I'm not sure whether the 5th/6th GPU aren't getting enough power or if they're having issues communicating because of the risers, but the other cards seem to be doing just fine.  Here's a pic of the rig