I like it. If this proposal was to come through, I can imagine a web app (or perhaps a feature add-on to say mint.com) that took into account what your salary was going to be for a given year, and calculated how much you could set aside for entrepreneurship without changing your tax situation.
It's worth pointing out that the UK already has these kinds of incentives, via the EIS "Enterprise Investment Scheme" that makes it so that investments in EIS-eligible companies (generally those developing new products or doing research) are deducted from one's taxable income for that year.
