Yes, same gun, same manufacturer. Already discussed awhile back on some 1911 forums. The only noticable differences are that the Sig has different style grips, different markings for their name, you're paying more for the Sig name, and supposedly it might come with a non-threaded barrel (per the link below) but who knows for sure about that. All GSG 1911's come with a threaded barrel, whether it's the 'tactical' model or the regular model. If the Sig does come with a non-threaded barrel, then I bet it will be interchangable between the GSG and Sig.
I'm sure extra magazines will still be expensive, since the cheapest I can find GSG mags right now (which will most likely work in the Sig just fine) is $29 each.
More pics here from SHOT 2011: http://www.sigsauerguns.com/tag/sig-1911-22lr/