Hi,
I am italian postdoctoral fellow under a J1 Visa (second year in US) working for the
Smithsonian Astrophysical observatory.
Do you think I am exempted for federal taxes?
For an unknown reason, on my 1042-S document I have a code 15 (researcher/fellowship) and I have exemption code 00.
I already did estimated payment for federal taxes for 2008.
Do you think I can use the Italian-US treaty for being exempt?
Do you think I have to attach my 1042-S to the taxes form?

Thanks

On J1 as researcher you are exempt from federal income tax for two year. You will file nonresident form 1040NR-EZ and 8843. Read page 16 of IRS publication 901: The Tax Treaties.
Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents

Thanks for the reply.
I am a J1 researcher but I don't teach. Do you think the tax treaty is still valid?
What can you tell me about the code 15 and exemption code 00 reported on my 1042-S? Do I have to attach the 1042-S to my tax forms?

You are doing research at the Smithsonian. By any definition, you are a researcher and therefore exempt.

Yes, attach the Form 1042-S to your tax return.