I purchased a new S2 the other day. Everything looks great so far, except that when I put in my existing SIM card, it only reads partial information (i.e., number, recognizes it is Vodafone, etc.) but fails to get register on any network - it stays in it's emergency calling mode. I've tried three other sim cards with the same results (all on different networks).
It actually seems like there's a problem with the sim reader or the network registration function, but I'm not entirely sure how to troubleshoot. The phone is "unlocked" - dialing #*7465625# confirms this.