To tell you the truth, I have tried countless amounts of texting apps for the iPod Touch before i got my very first iPhone. This is one of the apps i do not recommend.
The one i really recommend is TextFree. There is a paid version and a lite version.
the lite version is free but only lets you send 15 a day. on the other hand, the paid is unlimited.
The format of textfree is like the iPhones texting app.
You can send texts from textfree by inputting any number in the US and i think canada. For ex 1(510)xxx-xxxx or 1(808)xxx-xxxx
The person receiving your text can reply directly to the same text that you sent him, and you will receive it instantly.
There is also another option where you can input your cell phone number (THIS IS NOT REQUIRED) . It's just so that people can text your number, but instead of the text going to your phone, itll go to your iTouch. so it wont charge your phone.
i work at an Apple store and i used to work at a Mac store
i own a grand collection of iPods and iPhones =]