Here is some information from a few different sources. It looks like the bottom line is with a traditional IRA you must start taking the RMD. With a traditional 401(k) maybe not, see the details below and in the links to the original articles.
Traditional 401(k) -
Regardless of age, if you are still working you can continue to contribute to a 401K. As long as you own less than 5% of the business you are working for, you are not required to take RMDs. http://www.investopedia.com/articles/retirement/09/over-70-retirement-plans.asp
Required Minimum Distributions (RMDs) generally are minimum amounts that a retirement plan account owner must withdraw annually starting with the year that he or she reaches 70 ½ years of age or, if later, the year in which he or she retires.
However, if the retirement plan account is an IRA or the account owner is a 5% owner of the business sponsoring the retirement plan, the RMDs must begin once the account holder is age 70 ½, regardless of whether he or she is retired. http://www.irs.gov/Retirement-Plans/Retirement-Plans-FAQs-regarding-Required-Minimum-Distributions
When RMDs start
In most cases, the so-called "Required Beginning Date" is the year after you turn 70½. If you are older than that but are still working for the employer sponsoring the plan in question, and assuming you do not own 5% or more of that employer, you can further postpone RMDs until you retire. But the Required Beginning Date for traditional IRAs is always the year after you turn 70½ regardless of whether you are still working. http://www.workforce50.com/Content/Articles/CY9-Required-Minimum-Distributions.cfm
Hope that helps!
Yes. Your first RMD must be taken by April 1 of the year after you turn 70½. Subsequent RMDs must be taken by December 31 of each year. If you don't take your RMD, you'll have to pay a penalty of 50% of the RMD amount.