How do I get a certificate of US tax residence for my business? We are
working with a Greek company and before they can pay our
invoice, we need to provide them with a certificate of tax residence
that proves we are a US business.

Hi chazhull,

From irs.gov:

"Form 6166 is a letter printed on U.S. Department of Treasury
stationery certifying that the individuals or entities listed are
residents of the United States for purposes of the income tax laws of
the United States.  You may use this form to claim income tax treaty
benefits and certain other tax benefits in foreign countries. The
revised Form 8802 will streamline the processing of certification
requests by only requesting relevant information. The additional
information normally required with Form 8802 will no longer be
required when requesting additional certifications using the new
Additional Request box instruction. We expect that the revised form
will reduce the time it takes to process an application and decrease
the number of cases requiring additional information or
correspondence. This guidance supersedes the information printed in
the June 2004 Publication 686 pending the revision of the
publication."

Form 8802 is here in .pdf form:
http://www.irs.gov/pub/irs-pdf/f8802.pdf

You'll have to give a statement under penalty of perjury for a corporation:

"A Corporation 	A statement from an officer of the corporation with
the authority to legally bind the corporation stating:

"Corporation Name, (TIN) is a U.S. resident and will continue to be
throughout the current tax year."

Sources:
http://calbears.findarticles.com/p/articles/mi_m1052/is_n6_v111/ai_8948311
http://www.irs.gov/businesses/small/international/article/0,,id=122559,00.html

Search terms:
"irs certificate of tax residence"
"irs 6166"
"site:irs.gov 6166"

I hope this helps, and if you need any additional information or
clarification, let me know and I'll be glad to help.

--keystroke-ga