I was stuck in the exact same situation within the first month of my purchasing the MBP. I now have two prominent dents on the left side. Stupid kids make me go
Anyway, here's what I did and this, my friend, would be your safest way out of the mess.
Take your MBP to the Apple Service Center, pronto. Talk to the service guy in charge and tell him that you have inherited the MBP recently and would like to perform a complete hardware check on it.
Ideally, if you're nice to them, they would do it for you without any hassles.
Once they're ready to do it, report a minor problem (like speaker output being less than usual). They would probably record the external damage in (the form of dents, in your case). You will then receive appropriate documentation (a receipt of sorts). They'll probably keep the machine for a day or so.
When you return to collect your MB, you'll get a final feedback form and a record slip which you'll have to sign. Now, assuming that there's not any comprehensive damage done, you'll be safe for warranty for the remainder of your warranty period.
You can show them your initial slip which would be a proof that the machine was dented fairly soon after your purchase and there was not much damage done.
This is how it has worked for me for the past 2 years and I have been provided with prompt service (including the replacement of a hard disk).
Hope this ethically questionable method works for you.