I have international roaming data plan for my BB so I
could check my emails and use internet from any
country. If I unlock the phone and switch SIM card
with a local carrier SIM card from any country, will
that take away my data usage?

Generally local carrier SIM cards (i.e. prepaid sim cards) don't have Blackberry Data Plans on them. Without the BB data plan on the sim card, you'll get no email/internet.

However, if you have the phone unlocked you'd be able to use prepaid sims for voice/sms.

Can I use both the SIM cards simultaneously in my 8700c. i.e. cingular USA SIM card for data plan and prepaid SIM card for voice?

You can't use them "simultaneously" as there is only one sim card slot. You can remove the data one, insert the prepaid one and make calls and then put the data one back in. You can swap them out as you like (assuming the BB is unlocked.)

Thank You John! I did put in a request with Cingular USA to send me the unlock code but the support tech didn't sound too confident. Would you recommend a quicker way of getting the unlock code for my 8700c? I found bunch of websites offering this service, but was not sure if they were legit!

I've used GSM PhoneSource - When Only The Best Will Do many time with excellent results.