I'm having the same problem. MC17 worked fine on my two older iPods (80GB and 120GB models); MC18 would lock up as soon as I plugged in either iPod into my machine. I then ran down to Walmart to purchase a new 16GB iPod Nano - MC18 didn't recognize it either, and MC17 (I reinstalled it, per another Interact forum post) displayed popup errors that it couldn't recognize the type of iPod that was inserted (i.e. the iPod Nano).
How did I offload my songs onto my new iPod Nano so I could use it at the gym? I reinstalled an alternative MP3 manager / player, spent five hours importing my library, re-tagged the desired files and mission completed - that application automatically recognized every iPod that I own (and no, it wasn't a software app created by Apple, either).
For me, if MC can't recognize my iPods then it loses immense value.... hoping JRiver can immediately address this problem, as I love the sound quality of MC and I appreciate the vast power of it's tools, but the inability to easily interface with my iPod is a showstopper...