Oh man, what a day. Earlier today, I got my prepaid iPhone activated after a long struggle, which you can read about here, along with an easy solution. After all that, what time was it? Time To Hack!
So what did I discover? I found that your activated iPhone is a lot more flexible and powerful than AT&T and Apple admit. Pop out the SIM or put an inactive SIM and your iPhone works pretty much like a contract-free WiFi-enabled 6th Generation iPod. The WiFi means you can do Mail and Safari without a phone connection.
For your delectation, here's the stuff that I did and the results I discovered. I am still working on writing all this up nicely but I thought I'd rather share sooner than later.
Removed SIM and replaced it with an invalid, unactivated, no-account SIM. Still works as 6G WiFi iPod. Still Synced properly to iTunes. Tried placing call. It failed (as expected).
Removed SIM entirely. It complains but works at 6G WiFi iPod. Tried placing call. "No SIM card installed". Failed. Connected to iTunes. Synced without any problems I could detect. WiFi fine.
Removed activated SIM and placed into a cheap disposable AT&T cell phone. Worked fine. Was able to place calls. Good way to save your iPhone from danger on ski or bike trips.
Placed SIM from cheap disposable AT&T Pay as You Go phone into iPhone. Call failed. WiFi fine.
And remember: you still have to fund the phone and for those of you who are our friends across the border, I'm not sure how a Canadian credit card will play on the AT&T GoPhone funding site.
Conclusions: Buy the iPhone using prepaid. Activate. Cancel. Enjoy.
Want me to test something out? Let me know in the comments.