I came to US for the first time during March 21 2007, with my spouse and kid In l1b and l2b visa respectively. When I submitted w4 during April -2007. I claimed 4 allowance and marital status as married (one of the website guided me to choose the allowance). My employer replied me back saying only after six months you can claim married status as married and 4 allowance and asked me to re-submit w4 with single status and allowance 1. Which law says that I should wait for six months to claim married status and allowance four.
2.When we are filing jointly, does my spouse needs to the sign the tax returns.
3. if my wife and kid are not with me in US, can I still claim the marital status as married and claim allowance 4.

1) There IS no law. You can claim whatever status you want, as long as you recognize that you face penalties if you have not withheld enough taxes so that the check you write when you file does not exceed $1,000.

2) Yes.

3) You can file jointly with your wife for sure. You should also be able to claim your child as long as they stay with their mother when out of country.

Your employer is technically right. Non-residents should only claim one exemption. That's pretty much as it goes. You will actually be a dual status alien so you will be a little better off than a non-resident alien.

Your default status is dual status alien. Youl will not be able to file jointly unless you elect to be a resident for the full year -- this is an election however and not your default status.

I bow to TTE's greater knowledge in these matters.