Every text counts against your plan. Each incoming text counts as 1 msg; each outgoing text counts as one msg (as long as the text msgs are under 160 chars. Each 160 chars is one msg. So if you send a text that is 161 chars, it counts as 2 msgs!)
Only if you have one of the older messaging plans which included mobile to mobile texting would messages to/from other at&t customers not count against your message package allotment. These plans are no longer currently offered (IIRC they were discontinued February 20, 2008), so if you have one of the currently offered plans, then all messages are considered.
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.