Throughout the series, it is hinted that Iroh has visited the Spirit World. He could see Aang riding Roku's dragon and has a great deal of respect for balance and other Spirits.
My personal theory is that he went there to try to revive or at least speak to his son Lu Ten after he fell at Ba Sing Se, but AFAIK this is simply speculation on my part. I would, however, like to know if there are any facts that support (or disprove) my theory.
Is there any evidence as to Iroh's motivations for visiting the Spirit World or the time frame in which he did so?