To answer some of mr_boogie’s questions…
Taiwan source income is for income paid in Taiwan OR services paid outside of Taiwan but performed in Taiwan.
If you are in the country for less than 90 days in a year, you are not taxed on overseas income for work performed in Taiwan. Yeah, it’s kind of crazy. So if you were present here less than 90 days in 2005 then your overseas income will not be taxed. If you were paid in Taiwan though, you would have to pay tax at the non-resident rate.
If you were present for more than 90 days and less than 183, then overseas income for work performed in Taiwan is taxable at the non-resident rate.
If you were present for 183 days or more, then your overseas income for work performed in Taiwan is taxable at the resident rate. Also note that if you are present for 183 days or more, the tax office may try to claim that any overseas earned income was related to your presence in Taiwan unless you can prove it wasn’t. So it’s best to keep your mouth shut about this, even if that income is completely unrelated to your presence here.
And note that this is based on total presence in Taiwan in a calendar year for any reason, not just times you were here for business/work.