WOW definitely a surprising turn of events... Now, with TNA's more (or less) recent acquisitions of established older stars such as Nash, Sting, Steiner, Foley, Angle, it is quite a logical thing to think they had been gunning for Hogan as well. I'm just really surprised it worked out (because it must've cost them a load of money, but I suppose Hogan needs some after all the divorce issues and what not)... as for the real impact it will have on TNA, I don't know.
I think it's a nice thing that they have Hogan popping in, just for name value reasons alone, but there's no way he'll be around for any longer period of time whatsoever. And even though this of course looks a lot like WCW in the mid nineties before the great era of Monday Night Wars started, I don't think the same kind of thing will happen now between WWE and TNA. I mean, Hogan is THE man as he revolutionized wrestling not once, but twice... but come on, even Hulk Hogan is only human after all, his body is really worn and he is approaching 60 years of age... He just can't do it a third time, not even Hulk Hogan.
But I think that this signing will give TNA another bit of credibility boost, especially with an audience that has so far not paid too much attention to it. For even though TNA have already several big old time names on their roster, those with big name value from the old days (Nash and Sting) are just way past their prime, and Kurt Angle, as great as he is, never was the huge star that Nash, Sting or especially Hogan were. So TNA having Hogan on their roster, even if only for a short while (and I'll be damned, seeing the Hulkster and Sting in the ring again another time is still a great thing haha) will definitely be an eye-catcher. Bischoff being around isn't exactly a bad thing either... for say what you will about him, he has a nose for the business... and that is something TNA really needs right now, more so than even the huge name value of Hulk Hogan.
So, to sum things up: I don't think this means anywhere near as much as WCW's signing of Hogan meant back in the 90ies, but it is a bold move by TNA and it could shake things up a little, and maybe put TNA back on track... they have a good product, but just need bigger exposure to get people hooked... and Hogan being there could help that. Simple as that. So I'll definitely tune in and see what they do with Hogan.