you can send a special email and it will go through as a text, but you woldnt be able to recieve texts, a simpler way would be to use google voice, and skype would work well for calls the iOS4 will allow you to recieve calls anytime you are not in the app aswell.
I think the ipod touch is a perfect companion to a phone, specifically it can be very well with a android with wifi tethering you have the best of both devices and both app stores