I'll go for 45+ but not expecting a lot of + because:
It's supposed to be based on the current Insight. I get between 45 and 48 MPG on my Insight in realistic driving conditions (mostly highway, some hills, some traffic, and "I Can't Drive 55").
It is rumored to have a 1.5 Litre engine, the Insight has a 1.3 -> worse gas milage
Honda technology is awesome and they will find some way to improve it slightly
It's supposed to be sporty. Whatever they do to improve the gas milage, we will drive it to have fun.