Its: framework-miui-res.apk/res/arrays.xml - there's list of network operators and their GSM codes.
Unfortunatelly on some devices carrier names are read from SIM card and there's no sollution to change that.
E.g: on my Nexus S I succesfully changed my carrier name (from T-Mobile.pl to T-Mobile) putting my own network operators list in that file but the same on SGS2 doesn't work. It displays an old name of this carrier (ERA instead of T-Mobile). And there's no "ERA" string on decompiled MIUI rom.