A lot of people have been trying to get WhatsApp Messenger on WebOS, but so far no luck. *I've personally also tried contacting both HP/Palm and WhatsApp Inc, but again... no luck.
I've started a petition that will hopefully get the ball rolling:
WhatsApp Messenger for Palm/HP WebOS Petition
I think it's a great way to increase support for WebOS and it's products as well, seeing that the big 3 (iOS, BB and Android) all have the messenger available on their phones.
Thanks for your help!