I respectfully disagree with Eric.
If you invest from day one in a Roth, where, exactly will taxable income come from to put you in a higher bracket at retirement?
I agree, maybe for other reasons than you list, that early on, post-tax is the way to go, because I'll assume that one starts work in a lower bracket. There's probably a majority who start work in the 15% bracket, but over time work their way to 25%. As they grow to 25%, using the pretax 401(k) and IRA can keep them at 15% for some number of years. In any year of low income, for whatever reason, they can convert the pretax money to Roth to top off the 15% bracket.
To bring the point home, if you went post tax the whole way, imagine retiring with $2M in the Roth, but no pretax money. You now have a standard deduction, exemption, and the full 10% bracket each and every year gone to waste. Money you paid 25% tax when you could be paying zero on some of it. The deduction and exemption add to $9750 in 2012, and the 10% rate applies to the first $8700 of taxable income. If you do the math, this is $17,500, and if you plan a 4% withdrawal rate, you'd need 437,500 pretax money to give you the 17500 each year. Last, all three factors (standard deduction, exemption, and bracket limits) all rise a bit each year. An increase of $200 is another $5000 to save pretax.
Edit - 2013 brought the ability to convert within one's 401(k) from the traditional pretax side to the Roth side. This makes possible turning 100% of one's 401(k) money to Roth, and retiring without the benefite of the standard deduction, exemptions, etc.