We have not discussed this point, but I'm assuming it's off the table due to cost. And yes, this is for 24x7 rotations where everyone is in the same timezone...
http://blog.pagerduty.com/2011/03/on-call-best-practices-par...
This is a series of posts that have pretty sane defaults; I personally would not do a daily rotation, but rather rotations of 5 days, and alternating weekends (one guy does M-F, one guy does Sat/Sund) and you switch off.