As this BankRate article described, one can still tap into 401(k) account before age 59 1/2 without triggering 10% early withdrawal penalty.

Here is how it works (based on a complete FAQ from IRS):

1) One first needs to determine his/her life expectancy (say if you are 40 and your life expectancy is 45, which means you are expected to live to 85) according to IRS papers (more specifically, "[t]he life expectancy tables that can be used are (1) the uniform life table in Appendix A of Rev. Rul. 2002-62, (2) the single life expectancy table in §1.401(a)(9)-9, Q&A-1 of the Income Tax Regulations or (3) the joint life and last survivor table in § 1.401(a)(9)-9, Q&A-3 of the regulations.")

2) One needs to determine the distribution frequency (usually monthly, quarter or annually); distribution cannot be less frequent than annually

3) Then one can decide the periodic distribution using one of the three methods:

- Required minimal distribution method: one must withdraw total account balance / life expectancy in years every year -- annual withdrawal amount can be different based on year-end account balance

- Fixed amortization method: equal annual withdrawal amount is determined by an amortization schedule with the assumption of a fixed interest rate (which should be 120% of the "federal mid-term rate")

- Fixed annuity method: equal annual withdrawal amount is determined by using the total account balance divided by an annuity factor published by IRS

4) One usually cannot change the distribution schedule for at least 5 years, or until 59 1/2, whichever is later. Otherwise, 10% early withdrawal penalty may apply.

Source: MSN Money (it is only a short and brief discussion of the same topic)