I think it's only server-efficient if you make it ONE set time, and not give the user any time-options to choose from. Basically, give them this option:
Receive weekly Newsletter: Yes/No
Giving people monthly/weekly/daily options will cause a lot more mails and forces the cron job to be run much more often.
But other than that technical detail, I think it's a good idea, would be nice to send out an email to all members beforehand (all who have "Receive Admin mails" set to Yes, anyway) to inform them of the new feature, and the changes etc. on vB.org. Might get a lot of people's interests in vb.org back up, that