This used to work in version 4.6.10 but is now broken. To replicate this:
1. Create a contribution for membership.
2. Use a confirmation page.
3. Enable both a payment processor and pay later option.
4. Enter some pay later instructions
At this point, the contribution page will work fine with the payment processor. However, pay later instructions are not shown on the confirmation screen. Also, emails are not sent for pay later payments.
in version 4.6.10 pay later instructions are shown on the confirmation page and emails are sent for pay later options. They are not not in version 4.7.1 and 4.7.2
Other people agree this is a problem see http://civicrm.stackexchange.com/questions/9953/pay-later-instructions-dont-show-on-membership-contribution/9976#9976
I've only tried this on drupal7. apologies if I have missed anything here or got it wrong, it's my first time here...