while it would be nice if isshin wold just tell ichigo the truth, it seems rather unlikely. i, personally, think the only reason that ichigo would find out is by accident.

yes, it was hella-cool to see ichigo's father in his shinigami robes, his restrictions lifted and all, but it's been a secret from ichigo his entire life, something that even his mother knew but didn't tell ichigo about. i don't think he's going to openly admit to his son all of a sudden, even though he's so deeply ingrained in the world of the shinigami.

and in reference to byakuya's comments to jyuushiro, byakuya was talking about kaein, the shinigami that looked exactly like ichigo until he died, rukia's vice captain. and the shinigami that rukia compares ichigo to at the beginning of the manga and anime series.