Verdict: Apple is NOT withholding taxes from App Store developer payments.
https://marco.org/2009/01/29/verdict-apple-is-not-withholding-taxes-from-app-store
I just reviewed my records (with the excellent AppViz) and did the math, and Apple is definitely not withholding any taxes from my App Store payments. Not a penny. The deposits are exactly the quantity of unit sales times the price of the app minus Apple’s 30% commission.
I updated the Stack Overflow thread with my findings.
I’d love to hear from other iPhone app developers on this.