The only way for this to actually work is if you use the Multimedia Message instead of the standard text message.
What do I mean by this? If you have a cellphone number, most likely you already have email with that number. So the example would be is
123467890@provider.com and you just use your email from your iTouch or iPhone to send a email to your friends phone & let them know in the message that its you. Then if they have Multimedia Message enabled on their phone (which most phones these days include Multimedia Messaging as part of the service) then they can reply back to your email and you would get it to your email.
I had to do this when I went to meet up with one of my friends. I had no minutes on my phone, so I used my iTouch and everything worked out find since the place I was meeting them up with had FREE WIFI so I was able to stay in contact. You should try this out. If you have anymore questions, PM me and I'll get back with you with more detailed information.