Why don't you tell him a little white lie? Tell him you will be checking up on his texting activity and that owning an iPod touch is a privilege and not a right. Doesn't really matter if you do check or not- as long as he thinks you are checking.
That doesn't have to be a white lie; if you think you might have a need to check, it's the truth - and I think it's a good idea.
I understand the OP's concerns well, being the parent of a teenager. Being a good parent means finding a balance that allows the child to grow, but also allows the parent to parent. Parenting is not a popularity contest, and sometimes we have to set down boundaries that kids would rather not have. Not only that, but sometimes events transpire that give us reason to think something might be going on with our kids, and we have to find out what's up. It's part of the job.
I told my son I would respect his privacy in regards to his computer and phone as long as I did not have reason to suspect he was lying or getting into something that could cause him or others trouble, or compromise safety. I made it clear I WOULD check whatever I felt necessary if I felt there was a reason to do so. I suggested he ask himself "how would I feel if my mom and dad read this?" before texting, writing, posting or downloading something. These are the terms he agreed to, in order to be given the electronic equipment he has.
To the OP -
MacDawg is right, you're going to get a lot of responses from kids here. And of course they will see this as spying, violating rights, they'll comment on your parenting skills etc. You and I know better. Our job is to get them safely through this period of their lives, and this is one area we have to address.
Good luck.