I have been in relation with my girlfriend since the last 1 year. Her parents know about us since the 1st month itself.My father is not agreed about us as she belongs to a different caste and believes that her family's financial status is lower.Her dad wants confirmation from me within December 2010 but has no problem with this relation.

I am guessing you are not from a western country? That is not a bad thing, but it sure places a different spin on your situation. Not knowing what country you are from it is very difficult to offer advise.

I suggest you produce the paper work from her family financial status, or something, so your dad can relieve his suspicions, and you get confirmation for her dad.