My company now allows us to deposit into a 401K pre-tax. Is Dec 31 the last day I can contribute or does it extend into April for the current tax year?

Yes, December 31 is indeed the last day you can contribute for a particular tax year. Keep in mind that a 401(k) plan is a form of deferred compensation plan, and your contributions are deferred from your pay as you earn it. Hence if you receive a paycheck on December 31, 2008, any 401(k) contributions deducted from that paycheck are considered to be for tax year 2008; if you receive a pay check on January 1 2009 deductions will be considered as part of tax year 2009's contributions.