Just because mom's the breadwinner doesn't mean that dad is the default primary caregiver for the kids. Many dads these days work from home full-time. For some stay-at-home dads (SAHDs), daycare or sharing responsibilities with a grandparent might still be a better option.
Some stay-at-home dads (SAHDs) really get into the primary caregiver role - for them, it's just a way of life. Plus, there are many resources these days for SAHDs, including at-home-dad newsletters, mailing lists, bulletin boards, chat rooms, support groups, and even an SAHD convention.