The iPhone Dev Team, famous for releasing software that jailbreaks and unlocks Apple iPhones advised their readers not to use SIM card unlock or proxy SIM card solutions to aid the unlock. According to the team's blog, these "sim mods so far seem very very very questionable". The team doesn't provide deep inside regarding the reasoning behind their conclusion but warns people to avoid them.

A quote from the iPhone Dev Team blog:

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.

Source: iPhone Dev Team Blog