If You have not filed your retun till the due date of filling return then there are no penalties till 31st march (Assessing year),
If you have not filled your Return after Assessment year has been complated you will get penalty of Rs. 5000/- as per section 271-F....is you are lianble to pay tax on your income. (NO penalty for zero tax liability)..
If you have income on which you are not liable to pay tax for A.Y - 2010-11, You can file till 31st march-2012, but if You have income on which you have to pay tax..so After 31st July-2010 You have to pay your tax with interest u/s-234-A, and if you fails to file your return after31st march-2011 then you are liable to penalty u/s. 271-F.