Options for avoiding Private Mortgage Insurance (PMI)?

A couple of very attractive deals in the local housing market have prompted me & my wife to consider bumping up our timetable for buying a home (we've been budgetting for mid/late 2009 while aggressively paying down debt). If we bought now, our monthly debt+mortgage/gross income ratio would be under the magic number of 36 percent (by a comfortable margin), so it is feasible ... but we don't have a downpayment. That brings up the issue of Private Mortgage Insurance, which I would like to avoid. I read somewhere that invesements such as stocks can be used as collateral to avoid PMI. What about my 401K account and my wife's retirement account? Could we use those?