I am staying in USA for 181 days. I am working on H1B visa. Being paid a salary in US & federal, Social Security & medicare taxes are getting withheld from Salary. Is there any difference if I stay less than 180 days or stay for 181 days as planned rt now?

You pay taxes... the rule took far more days out of the country to get a break for the year then that (it only allowed about 30 days in the country during the year), and in any case that only applied to state and local and some federal taxes. SSI, and Medicare are never exempted. I've actually been in that situation.

Unless of course you manage to get yourself elected to office... they exempt themselves from what the rest of us have to pay.

Thanks for the quick answer.. I appreciate..
Just to conclude & understand what you say, plese reply if the following statement is true or falls.
In the situation I am right now, its irrelevant if I stay in US for less than 180 days or for 181 day

The Substantial Presence Test nominally lasts 183 days, but is also counts days spent in the U.S. in the previous two years on a pro-rated basis.

So, if you will spend only 181 days in 2010, and you spent NO time in the U.S. in 2008 or 2009, you have NOT met the Substantial Presence Test and will file as a non-resident alien, filing Form 1040NR.

Now, the advantage of filing this way is that, under IRS rules, your visit is considered to be temporary in nature, which means you can claim daily living costs (rent, utilities, local transportation, food and incidental costs) for the 181 days you spent in the U.S. These costs will be claimed under Form 2106 and will be added to the Schedule A page on the 5-page Form 1040NR.

Go with what AtlantaTaxExpert says on this... the laws on this I was familiar with have apparently been superceded in the interviening years since my first hand experience with this. I squeeked by on the exemption by as little as only several days in my case (4 times this has applied to me) but that was long enough ago that the tax code has apparently changed. And they are updated at least annually.