Latest attempt to get some info on this, auto response from WhatsApp after I asked them about the N9:
"Sorry, your device Nokia N9 is currently not supported and we have no plans to support it at this time. We currently support iPhone, BlackBerry, Android, Symbian S60, Windows Phone 7 Mango and Nokia s40 devices.
I'm sorry that we have no further information to give out at this time. Thank you for feedback on this matter.
Thank you for contacting WhatsApp."