I am in US from 27th Aug 2010 and getting per day expense from my company. This is not as a salary, I am still getting the salary from my same employer in India. With this arrangements, I am not paying any tax to US. If I plan to stay for more than 183 days may be for 10 days more would I or my company needs to pay any tax to US government? I am expected to travel back to India but I wish to be on PTO (leave) for 10 more days (after 180th day) to see places around in US. Can I do this? Or I have to go back to India on 183rd day? Any help would be really great in this regards. "

If you stay beyond 183 days under EITHER the L-1 visa or a tourist visa, you will meet the 183-day Substantial Presence Test (SPT) and your Indian salary and possibly the daily per diem will be subject to U.S. income taxes.

However, note that the 2010 days is reduced by two-thirds under the SPT formula, so, for SPT purposes, 2010 counts for ONLY 42 days, which means you can stay 140 days (about 19 May 2011) without any tax consequences.

Cool, so does that mean I can stay here till 8th March 2011 easily without affecting my Indian or US perdiem for tax and not affecting my employer for taxation as well?

Just wanted to reconfirm this :) thanks.. it is definitely turning out positive

Yes, no problem! By my calculation, you can safely stay until mid-May 2011 without meeting the Substantial Presence Test.