I realize that this doesn't help right now, but we always fund our Vanguard IRAs electronically.
I have Vanguard's "maximize" contribution option enabled for my wife's IRA. Whatever the IRS max limit is, it'll auto split your available left for the year over the number of remaining contribution periods. So if you had turned on 01/01/07, it would have auto withdrawn 333.33 or 333.34 from your account every month. And then autostep up to 416.66/416.67 for 2008 ($5K limit).
One option that would have been possible for those w/o electronic avenues is to go to a bank and open an IRA CD for 3 months and then have your fund company/brokerage roll it over upon maturity.