By default, the whisper will be published to your friends and fans only. Now you may also choose to publish the whsper to the front page for open anonymous discussion. You may also choose not to show this whisper to your friends and fans to make it harder for others to guess your identity.
There is now an option to post new whispers to the anonymous forum using a button on the top right: Whispers made from the anonymous forum will only be posted to the forum by default and not to your friends but you'll still be able to edit that setting before posting.
Addendum: It took some time but it's finally done! BigAndroid (the plurkwork in charge of android app) is still working on this so please give him some time. Now if you'd like to just hold a discussion with plurk strangers, you can plurk to the anon forum to avoid detection. Or just whisper to your friends for memes!
If your timeline is public, two options will pop up when you whisper:
Whispers made from the anonymous forum will only be posted to the forum by default and not to your friends but you'll still be able to edit that setting before posting.
Now if you'd like to just hold a discussion with plurk strangers, you can plurk to the anon forum to avoid detection. Or just whisper to your friends for memes!