I have and e61 with a Rogers voice plan, but not data plan. It is unlocked and I use Iskoot, fring, mobivox to access my skype account. As long as I can connectd to a WLAN wifi connection, I can surf the net, get e-mail and so on. I would imagine that you could do all that without getting a voice package but there are advantages of getting even a basic one, for 20 dollars a month...the one being that some services like require you to have a mobile phone number.
I would try to see what you can do without a voice plan.
Make sure that if you do get a voice plan, go to a private dealer who sells the plan and unlocked phone.
The e61 is a great machine and I am quite pleased with it. It is really a computer that you can make phone calls with.