Q: my security code is correct; i have checked it 5 times and it has worked for other vendors, but i still get an error indicating it is invalid.  what gives?

Is the address on your iTunes account exactly the same (format and spacing etc) as on your credit card bill : http://support.apple.com/kb/TS1646 ? If it is then you could try what it says at the bottom of that page :

If the issue persists, contact your credit card company and verify that they and any company they use to process credit card authorizations have the correct information on file.

And/or try contacting iTunes support : http://www.apple.com/support/itunes/contact/ - click on Express Lane, then iTunes > iTunes Store