Thanks for your question.
If you paid all the tax by Jan. 31, 2010, there will not be any amount for penalty/interest charged but you should file the return asap.
If you owe tax with your returns, there will be a late filing penalty and late payment penalty of a combined 5% on the amount of tax paid late for up to 1 month late. Interest is charged at a rate of generally 4-5% per year, so for 1 month late, for example, it would be about 4%/12 = 0.33%.
Also, if you were required to pay by federal tax deposit and did not do that timely, there could be a late deposit penalty charged ranging from 2% to 15% of the amount not deposited timely.
Hope this helps,