I have been an H1B visa holder since Aug'05. I had recently received a promotion (Sep'07) and a $4,000 sign-on bonus, which after taxes was $2884.58. On my W2, "Wages, tips, other compensation" is 2884.58 less than "Social security wages" and it appears like the sign-on bonus tax was not in the Federal Income tax withheld but was in Social Security Tax withheld. Is this normal or was there an HR or IRS reporting error? What should I do from here to claim tax returns on the bonus? Can I claim returns on Social Security tax also? I am filing 1040EZ (single-no dependents) for my federal returns. Please advice.

That is NOT correct. Contact your employer to get a corrected W-2.

The bonus should be subject to income as well as FICA taxes.