Well I don't exactly have an iPod touch with 3G data, it's an old iPhone 4 with an AT&T data only SIM card, and it works. Although you can't make calls or place SMS messages you can use the device basically as a faster iPod touch with a great camera and 3G data. 

The process was a little painful to learn since AT&T basically laughed in my face when I told them the idea. With a little help from the MacRumors Forums community and an iPad 3G from the Apple store I was up and running in no time. We will have a tutorial for those that want to know how in a short time. 

Everyone knows that an iPod touch is an iPhone without the phone but the iPod actually features a lot less than that. For example, I can use the iPhone's great camera, GPS and take advantage of the wireless chips. In addition, the iPhone has a pretty good flash for taking pictures and using as a flashlight. 

Speed over AT&T's 3G network seem inconsistently consistent with other iPhone 4s, as expected. Speeds were up and down but as I compared with another iPhone 4 that seemed normal. The top speed I recorded was 2.91 Mbps down and 1.27 Mbps upload, I had four bars when doing this test. 

For those that don't know, even when you have a data only SIM you still receive a phone number, although you cannot make or receive calls or SMS with the number. What you can do is FaceTime and iMessage with that number. The iPhone recognized that I did have a phone number and when people called via FaceTime or used iMessage to that number it actually worked. So when you do this technically you have a phone that others can reach you on. Of course you can setup FaceTime and iMessage with an Apple ID so you can use an email address also. In addition to having your AT&T number, you can also make and receive calls from any number with Google Voice or Skype. 

For what it's worth this will cost you monthly from a provider. I use AT&T and receive 250 MB data for $14.99 per-month or 2 GB for $25 per-month, both plans are month-to-month. In the end this is a great idea for those who want data on the go and don't have a smartphone. Buying a used iPhone and using it this way isn't such a bad idea but I can't see myself paying monthly for an iPhone plus another iPhone. If there was a tethering or hotspot option this may be a different story.