I've stared at IB also but Java serialization costs would seem to me to outweigh the improvements. I don't think you'd see much difference over gig-E. You'd need a very lightweight layer between IB and the application to reap the benefits.
Pathscale was around the 7us wasn't it?
I saw latency numbers at around 1.5-3 microsecs, depending on the distance between boxes. Yes, Java serialization would definitely be a factor, so you'd essentially have to get a buffer from the user, and avoid copying all the way down into the network card's buffer. At one point, the buffer has to cross over into the C heap (typically with JNI, but I understand there are other, faster options), so there's a performance penalty, too.
I don't know how approaches such as R/DMA can solve that problem, and I haven't seen numbers on how much faster they'd be, compared to regular JNI.
Googling this topic, I understand SUN once looked at providing RDMA (or SDP?) directly in the VM, but dropped it because of lack of interest. Damn, they didn't ask *me* ! :-)