PS Paging is a mandatory message.
It has the same purpose as CS PAging : a MS can be paged by the entwork in order to establish a GPRS connection.
Zero packet paging means that all users are establishing their own TBF. There is no case in which a MS is paged by the NSS in order to open a data connection.
Now, if there are low bandwidth / long time connection (such as MSN messenger), during which the user can remain inactive for quite a while, until somebody sends him a message –> that could lead to GPRS paging.
Same thing with Push to talk, I assume the called party will be reached thanks to a GPRS Paging.