Alright, so 420 is out...we're all working on our blogs, discussing the chapter...and we've finally learned that FGT is a form that will ultimately cause the loss of Ichigo's shinigami powers. If you pick up the pieces and look at them, it seems that would be the logical explaination for Isshin's loss of his own powers for so long...but he regained them. It seems like he wouldn't know jack about it if he hasn't used it...sooo...it's not speculative to assume that he did.
The question that arises in my mind...is what would have forced him to use the technique? It couldn't have been Aizen, it would have had to be some threat that was strong enough that he'd have to go passed Shikai (and Bankai, as we'll assume he possesses it) to actually become Getsuga (as per the Final Getsuga Tensho). And furthermore, whatever he fought...did he really kill it? He may have defeated the threat but...something tells me if we're in for a new antagonist, it has something to do with Isshin and we'll probably know about this individual whenever Kubo explores his past.
Of course, while unlikely, Kubo could have it that Isshin used the technique merely to see what it could do...and accidentally lost his powers because of it. I imagine a comic relief segment following this explaination but I DOUBT THAT'S THE CASE, so there was some threat at some point that would require him to go into the form. So...what do you guys think? Did he really kill whatever it was that required him to use the technique or is it going to return as the new antagonist or at least a threat in the future?