What is the deadline for investing in a 401K for 2008?

Dec 31, 2008. Remember, a 401(k) is a deferred salary plan, so your contributions for 2008 are from deferred salary you earned in 2008.

I'm wondering if you actually meant to ask about IRA contributions, as opposed to 401(k) plan contributions. The deadline for making investments in an IRA for tax year 2008 is either April 15, 2009 or the date you file your 2008 taxes, whichever is earlier.