This hack is requested so much but I cant really see why it's so much requested..
IMO it's much better to mass email members (which is a default vb feature) than mass PM.
If your Pm is 3 kb. in size and if you have 10000 members, 1 mass PM action will cost you 30 MB. of server space and db expansion. The replies users will produce is not included to this loss..
In this example it will create an extra 10000 to 50000 query in your server at the moment you triggered it.
Email will be read by the users very quickly, while PMs is accessible only when user logins to your site.
It will create problems if user disabled PMs or has a full PM box or just dont know how to use them.
It will create mass reply chains..
If I were you I would really consider this feature twice before using it. I would say go with mass emailing or dynamic forum home announcements hack..
My 2 cents..