http://thefinancebuff.com/obamacare-3-8 ... x-amt.html
By the way, your statement isn't correct. The 0.9% tax is on taxable wages over $250,000, not total income. For example, a couple with $260,000 wages and $20,000 investment income will probably not pay any of the 0.9% tax, since their pre-tax insurance premiums and FSA contributions can get their wages under $250,000, or very close to it.
As for the 3.8% tax, it is not on unearned income, it's on certain categories of investment income. Muni bond interest is excluded. Furthermore, the threshold here is not total income or wages, it is AGI. The example couple above would probably not pay any of the 3.8% tax, either, because 401k contributions, FSA contributions, and pre-tax health insurance premiums could easily drop their AGI to under $250,000.
One ACA tax that is "absorbed" by the AMT is the increased threshold for itemized medical deductions. Since taxpayers in the AMT are already subject to the higher threshold (10% of AGI), this new tax affects only taxpayers who are below the AMT.