I am a US citizen who has lived in Thailand on a yearly-renewable non-immigrant business visa for the past 5 years. I have been religiously paying my dues and filing my Thai income tax returns but was not aware until recently that I have to continue filing in the US as well. I have also received a couple of W-9s from Thai establishments which I am at a loss as to how to fill out. I no longer have a US address and am working on getting permanent residency in Thailand. On my Thai returns my address is of course my home in Bangkok, but what address should I enter into the pertinent fields in the W-9s and the US returns (past and future) I intend to file? At this point, because of my ignorance, the address on file with the IRS would be my last US address.
You should fill your current address.
On the default W9 form from the IRS web site, it says "City, State and Zip code", but nothing prevents you from writing "Bangkok, Thailand, WhateverPostal-Code", right?
Same for the tax returns, the IRS form 1040 actually does have designated spaces for foreign addresses.