I'm also doing this now. Although I sometimes consider going back to full time, the whole progressive taxes thing makes it seem like it's not worth it (but don't tell the economists that they're right!).
Progressive income tax is a good point, but won't by itself rationally affect your decision if it will take you more than about 5 years to retire working full-time. Not sure about the US but here in the UK the biggest proportional jump in marginal tax rate is from 20% to 40%.
Assuming a 7% real rate of return on the stock market, if the choice is between earning an extra $1 now at a marginal tax rate of 40%, and earning that extra $1 in 5 years' time at a marginal tax rate of 20%, then $0.6*1.07
5 > $0.8 and you're better off paying the higher tax rate so that you can benefit from stock market returns for those 5 years.
Also, if I worked full-time instead of half-time there's no doubt I could earn more than twice as much, even after tax, though it wouldn't necessarily be doing something I enjoy as much.