I have a question about a tax treaty between italy and the US.
I know that professors under a j1 or h1b visa are exempt from taxes for a period of two years.
I have entered for the first time the us on September 2006 under a j1 and moved to a h1b on July 1 2008.
I have two questions:
a. I think I should be able to apply the tax treaty until the end of August 2008
During my j1 period, I did not receive any compensation for 6 months (my appointment was a combination of a professorship for the first 16 months and then visiting scholars for the last 6). Since I have not received compensation for 6 months, can I extend the tax treaty with the h1b until February 2009?
Many thanks.

When you are on H1-B, you are on H1-B. So it will be H1-B treatment.
When you are on J1, you are on J1. After switching to H1-B, you are no longer on J1.

The answer to your question is NO.

Once you switch to the H-1B visa, you become liable for all the associated taxes (income, Social Security, Medicare).