I've been thinking along those lines, I might even hit the 401k hard and cough up a full 20%--but I don't relish the penalties.
See if your plan will allow for you to withdraw in the form of a loan that will allow you to pay back over the course of several months (12-60) and therefore avoid the penalty withdrawal. Our 401K has that option...you just select the amount that is withdrawn from your paycheck that is comfortable and it gets automatically deducted each month to "payback" the loan you took out to purchase your home
Unfortunately, I no longer work for the company the 401k was with (and my 401k with my current company doesn't have near as much money in it). Theoretically, I might have been able to shift the old 401k to the management of my current company and then take a loan on it, but there just wasn't time.
Instead, I converted to an IRA, took the 10K with no penalty, and took an additional 18,400 with the 10% penalty. I'm not loving that, but having the cash on hand gave us leverage for our bid, which was accepted despite being $28,000 below a competitive asking price.
We're going conventional, 5% down and 3.375% for 15 years.