I actually believe that we will eventually eliminate an income tax and move to a sales tax or value added tax (VAT). If we do develop such a system, I am sure there will be special provisions to tax dispursements from IRA's.
This is why I have funded a Roth IRA during residency (when I am eligible to fund one) in addition to my 403(b). I will fund a SEP-IRA or Individual 401(k) while working as an independent contractor next year, but I also plan to keep a significant number of investments -- mainly stock, which have lower tax rates -- in regular investment accounts. This will also allow me to have income when I retire early (hopefully at 50) since IRA's can only be accessed at 59 1/2 years of age unless you want to pay a high penalty.
When I move on to a fellowship in 2 years, I will probably fund the Roth IRA again. I would love to be able to rollover my 403(b) to my Roth, but my income this year will be over the limit.