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Zf_ from the iPhone Dev Team said the following:

OK, a first report on those method after some tests done by w___ and me, it works in a non reliable way, and is highly network and timing dependent (meaning that even on the same network, it can work sometimes, then not on the next try, and so on)

in summary, don't bother, wait for the next better method

Also, this is posted as a warning in iClarified:

We’ve been monitoring the whole “SIM-card unlock” proxy-SIM situation. This involves using a chip that is attached to your SIM card (with a small modification to your SIM) or sometimes a small PCB soldered inside the phone. These sim mods so far seem very very very questionable. Zf has found that they use trickery of the GSM and UMTS network that is considered highly illegal in most countries and they rely on sending bogus IMSIs and various other nasty hacks to obtain service on your iPhone. A couple of our members have worked out how this all “works” and we’ll try to publish our findings soon. DevTeam recommendation: Steer clear, don’t use!

So my analysis on this is that do not try to use them or even buy that solution. Illegal sound bad enough as it is just to find out once you buy the darn thing, it won't work.
 

Avoid these products at all cost! I'm kicking myself now but I should have read this thread before buying a Sim Card adapter from mooglex.com. I was mesmerized by the videos on youtube of both the Rebel Sim Card and the very similar product from mooglex.
While the adapter does indeed unlock the iPhone 3G (I could briefly use a TMobile Sim Card), there is a CRITICAL design flaw. The added thickness of the adapter + your Sim Card causes the entire package to become stuck in the Sim Card tray. As a result, I had to pry the tray open, slightly damaging it in the process. A simple Google search (which I omitted to do as well, duh) will reveal that this is a typical problem.

The worst part is that the disreputable people behind mooglex.com have ignored my email requests for a refund despite a promised 15-day money-back guarantee on their website! Avoid like the plague (in the pre-antibiotic era)
 
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