No, you can't unlock the Marbl from Virgin Mobile. It's so cheap and slow that you wouldn't want to unlock it first off, and second, it's hard to get a CDMA phone, especially a Virgin Mobile or Sprint one, to register on a network "away from home".
And yes, you could very well be able to unlock a Nokia 6061, provided you find the fight unlock code generator (not sure if this works) or cable (just find the right one) and software (find the fight one). GSM phones are relatively easy to unlock, you just have to find the right info and you're good. But, personally, the Nokia 6061 isn't a phone to spend too much effor unlocking, what with the T-Mobile Nokia 6101, which T-Mobile will unlock after 90 days of active T-Mobile To Go service with the appropriate excuse, available for effectivel $25. And that phone has a camera...
Hope this helps!