I'm going to play the bleach supporter card.
Isshin met Ichigo's mom (Masaki) after he used final gentzunga and lost his shinigami powers. she saved him by sharing some power with him, and eventually, by letting him (a shinigami that lives on power he draws from whats besides him) to smooch on her spirit power (which turned him human) and she lost her quincy abilities. when she died, the seal on Ishhin was lifted (I dunno how, magic or something) and he ragained his shinigami powers. Ichigo didn't die from using getzunga tenshou because he was born human and not shinigami, so he has some innate spirit power.

even if all that is crap; I still think that they met when Isshin used the final form.