Thanks for noticing and reporting back
.
Confirmed bug of profileBook with annonymous entries (of guests) allowed.
If this is not allowed, the notice isn't shown.
to fix the notice, just add:
[code:1]$curruser = null;
[/code:1]
at the begin of function getDisplayTab in the cb.profilebook.php file.
It's now fixed for next profileBook version (native).