I'm curious - is there a reason that the current models couldn't hold a 9 speed rear drivetrain now? I'm sure cost is a factor; it seems like 8 speed stuff is fairly cheaper than that. But I've also heard that chainline becomes more of an issue on a folder because of reduced wheelbase, and also it's a real 8 speed drivetrain, whereas in most bikes only 7 of the 8 speeds would be recommended for each chainring.

I guess the current freehub will hold a 9 speed cassette since they're the same size as an 8 speed.

Which chain are you spec'ing for use with 9 speeds? Something more bendy than normal? I've read the SRAM chains are a bit noisier than the shimano (9 speed) chains, implying that some chains are a bit better suited for bending than others.