After unwrapping all the plastic stuff, I popped the 3G SIM holder (it's a lot smaller and more fragile than the sturdy 1st Gen version) and plopped in a fresh PAYG SIM. The SIM had a 30 day/"Unlimited" data (i.e. up to 5GB) feature plan attached to it, which I put on there a few days ago, and otherwise has about 4 dollars and change of credit. (See my previous TUAW discussion about Prepaid Data Packages and the iPhone.)
After about two minutes, the iPhone acknowledged the SIM and threw up an AT&T logo. I made an outgoing phone call without any problems.
Getting the 3G signal proved harder. I had to walk outside until I found a 3G hot spot (and yes, I do live in a major metropolitan area). Once I got my bars and the 3G logo, I found the connection worked great. Nice and fast. This is the same SIM I had in a 1st Generation iPhone yesterday with EDGE.
I did not pwn the iPhone. I did not unlock the iPhone. It. Just. Worked.
If you're looking to use your phone data-only, you can sweeten the deal by taking advantage of one of AT&T's refurb packages. This Nokia deal comes with a $100 refill card for just $80 shipped. That's 5 months of 3G data for $80 and the credit lasts for an entire year. Not bad. You can donate the leftover phone to protect victims of domestic violence.
Make sure you add your next data feature packs before your 30 days expire. Buying before the end of the month extends the current feature pack for 30 days.
Should your feature pack expire, you'll be charged at a very high per-kilobyte rate. Call 611. Say "Buy Features". Which store? Say "Media Net". Listen to the three choices. Say "Unlimited" and then "Buy It". Quit with "I'm done here." You can "Check my feature packages" by calling 611.