I was just reading this thread and I was wondering if it's possible that Isshin did to Urahara what Ichigo did to Rukia? That is, taking his abilities. I mean, if this is so, then Urahara could have trained Isshin himself, which then explains why the other shinigami don't know who he is. After all, having history repeat itself isn't uncommon in animes. Naruto, for example, has Team 7 following the footsteps of the Sannin.
The only problem about this theory is the captain's coat Isshin had on his shoulder though. But then again, maybe Urahara gave it to him. We haven't seen Urahara wearing his coat before.
(p.s. this is my first post. please don't be too harsh if my theory is way off the mark!
)