I quite like the O2 method,
You can't enable or disable it, but instead type (star, zero, hash, space) before your message.
It then replies to you, spoofing the person you sent it to's number saying "message delivered from [your number] to [their number] at [time]. Message: [what you sent]"
The *0# isn't included in the message they receive.
This means it's optional. It's also free.
However, I'd prefer if it was a code that was more simple to type in text messages (instead of going through symbols and numbers). You can get round this with templates, but with my Samsung, they are pretty poorly implemented as you have to select the template, then press again then press send. (or from nothing on the screen, press menu, 5,1,4, select template and click, 2)
Do other networks send them in a similar way, or do they get sent as those messages which appear once which tell you your credit?