SMS was designed for text-based communications over cell networks. There is a reason SMS was originally 160 characters. It is talking to its cell phone tower over a pathway called a control channel. The control channel also provides the pathway for SMS messages. When a friend sends you an SMS message, the message flows through the SMSC, then to the tower, and the tower sends the message to your phone as a little packet of data on the control channel. In the same way, when you send a message, your phone sends it to the tower on the control channel and it goes from the tower to the SMSC and from there to its destination. Anything outside of that is being switched. Eventually SMS will die off as IP networks take over with the voice channel for phones being over IP as well. We're seeing it now with Apple's iMessages. iMessages aren't SMS simply because they are going between phones just as emails aren't SMS simply because they are text between phones.
if a featured phone that only has sms and voice can receive sms from google voice, I would consider it sms.
SMS is still only 160 characters. Servers/phone break it up for you automatically.
Lets not talk about iMessages as it requires data on both ends. Its no better than a chat program. Its irrelevant.