Brazen Careerist says you can quit work for awhile to have kids and your career will be just fine. She makes some really great points in this posting. I would also add two of my own:
1) Have a plan. You don't need to have every detail worked out, but have a general idea of how you might ease back into the workforce when the time comes. Will you stay involved in professional associations? Read trade journals? Take a few classes? Whatever it is, having some basic ideas about what is going on in your career will help you avoid the occasional moments of "What have I done to a perfectly good career?!" panic that plague most women that transition from career to at-home status.
2) When you do apply to return to work, be honest about your skills. I get concerned when I see advice to at-home moms that they can sell their PTA committee experience as management experience in a job interview. The fact is that running the PTA carnival does not equate to managing a department of full-time employees. The former definitely requires communication, budgeting and persuasion skills. However, it does not equate to the day in/day out 8-5 schedule of coaching direct reports on their career development goals, writing and delivering annual performance appraisals, and preparing Board reports; not to mention having the deep industry knowledge that is typically required when you move up the chain of command.
I am NOT trying to be discouraging to at-home moms who are returning to work - I am simply saying that you will interview better if you demonstrate a comprehensive knowledge of the environment you are seeking to enter, how it differs from the environment you have been in, and what qualifies you to make that transition smoothly.
Stay focused on these elements of your career, and you can definitely enjoy your time at home knowing that your career will be just fine.