I just encountered some problems with USB using r30089 on my 5G iPod. USB connections and disconnections are sometimes ignored, and USB disconnections are sometimes seen as USB connections. This once led to a lockup. When I got a connection, it was often at USB 1.0 speed, and once a USB 2.0 connection was reset, leading to a new USB 1.0 connection.
I use one of those USB + FireWire cables, using the FireWire end for power and the USB end for data transfer. This is because the USB power input on my iPod is broken. I wonder if this has something to do with the problem I observed, but I never observed this problem in the past.
I'm not 100% sure it's a Rockbox problem because recently I used my iPod rarely and its USB functionality even less. I guess the problem could just be dirty contacts. It seems like r27589 (which I had on my iPod as a known-good backup) didn't have any problems once I turned off the iPod and turned it back on, but maybe I was just lucky.