iMessages vs Texts

I thought that the default for iMessage to be sent was over WiFi when available. So I sent a bunch to my daughters, from home, where we have Airport. Every single one went as a text message over the cell system, and I was charged for every one sent and every one received--it really jacked up my bill. Is there any way around this? No chance I'm going to use iMessage much if I'm going to have to pay for every one. No different than sending a regular text. I thought that was the point of iMessage--no charge.

There is no default. All you need is an internet connection - via an available wi-fi network or via the cellular network. When connected to the internet via an available wi-fi network, an iMessage will be sent via the wi-fi's internet connection. When connected to the internet via an available wi-fi network, an iMessage will be sent via your carrier's internet network just as when sending email when connected to your carrier's internet network.

If a message is sent as an iMessage - the color of the message bubble is blue instead of green and the message should not be charged as a text. If a message is sent as an iMessage, it won't be charged as a text since it wasn't sent via your carrier's SMS/MMS system.

Go to Settings > Messages. Is Send as SMS on if iMessage is unavailable, and your daughters have iMessage enabled on their iPhone, iPad, or iPod Touch?

My wife and I have the same issue. We both have 4S's and it is not consistent between imessage and text. Wifi, 3G, or any other service and the majority of the time we are sending text messages. This started around May 14. Prior to that the large majority of ours were imessages. Since then predominately texts.. I don't get it. We made no other changes.

Only issue is she in on Verizon and I have AT&T. I figured it had something to do with that until I read this thread. Now I have no idea..