GSM Carriers (from GSM to GSM) are able to plop all the pieces into one message. The break happens when a CDMA carrier comes into the equation. They, for some reason, don't have the capability of doing this.
Keep in mind though, all the information above is still true. So even when a message from a GSM carrier goes to another GSM carrier and shows up as one long message, you still get charged for the individual 160 character messages. It's just seamless for viewing.