In bb-includes/class-phpass.php on line 43, bbpress uses getmypid() to create the randomstate for password hashes.
On shared hosting systems, getmypid() is disabled very often.
I replaced it with uniqid(), and bbpress didnt throw the WARNING anymore.
getmypid() does not return a unique value in all cases. Especiall when PHP is running as an apache module in shared hosting sytems. Also for this reason its much better to use uniqid().
uniqid() gives a 13 digits value. uniqid('', true) even 23 digits. I think this should be good enough for the randomstate. ;)
Please consider to use uniqid() in future versions of bbpress.
thx for your attention
ix