I worked in 2008 on H1B visa for the complete year. I got married in India and my wife came to US in May'08 on H4 visa. In August '08, she started working on L1B visa. Can we file Federal return jointly as residents? If yes, then does she have to report her income from India for Jan-Apr time-frame?

Yes, you can file joint return.
You must both declare your worldwide income for 2008. If you have any foreign income, and on that income you paid taxes in the foreign country, then you can claim foreign tax credit (Form 1116). Read: Your U.S. Tax Return: U.S. Citizen or Resident with Foreign Income (http://taxipay.blogspot.com/2008/03/us-citizen-or-resident-with-foreign.html)