Yes. WhatsApp has been pretty aggressive with hunting down and stopping any third party implementations out there. They're also issuing temporary bans for anyone caught using third party clients:
Reminds me a bit of the old dispute between AOL and GAIM, which was later renamed to Pidgin in part to appease AOL. WhatsApp seem to be acting in an even more aggressive way than AOL was, though. AOL limited themselves mainly to: 1) complaining about the name; and 2) playing occasional games with the protocol to break third-party clients. Afaik they didn't sue anyone claiming that simply distributing a third-party client was illegal.
There was recently an article from a person working at Microsoft who implemented AIM support in MSN Messenger back in the day who had similar experiences.