actually a lot's happened in the Paris arc. It's only been the last volume or two which hasn't done much between chiaki and nodame.
Chiaki saw his dad again and got fuck-all pissed
Chiaki admits to himself he actually does like Nodame
Chiaki and nodame have a huge fight because *he's* pissed she's not taking their relationship seriously
At the very least they've made out a few times, and it seems likely they've had sex and just haven't mentioned it to anyone.