I am filing combined tax returns with my wife. Both of us were on H1B. I am on H4 since August this year, will my wife get dependent's tax benefit for this year even though its just for part of the year. Also if I want to switch back to H1B again, Would it matter if I start from Dec or January 2011

If you are filing jointly, it will have NO effect at all on your tax return.

A joint return is a return of a couple filing as ONE tax entity, so the IRS does not care who is the dependent and who is the primary income earner.