It'd be an interesting development but, all in all, it wouldn't have the devestating impact on WWE that some think that it would.
If anything, The Rock signing with TNA and coming back to being a full time wrestler would only serve to strengthen the notion that TNA can't get over without ex WWE guys. TNA has built it's company for most of the past 5.5 years around former WWE and/or WCW guys and have been moving away from that image over the past several months. In some ways, signing The Rock would be a massive step backwards because you know....c'mon, you just flat out know that The Rock would be carrying the TNA WHC within 4 to 6 weeks, would decimate the competition and would immediately be elevated to being the face of TNA.