From halo.wikia.com (original source being one of the Forerunner novels)
"Forerunner" is a literal translation of the species' name for themselves; they identified themselves as such because they believed that they held an impermanent place in the universe's Living Time and that they would be succeeded by other, better races.

It's possible that Guilty Spark was saying that humanity, as the Reclaimers to the Forerunner legacy, would eventually become the same as the Forerunner: passing on the Mantle (or sumpin') to a future species.

: Applying that Spark and the IsoDidact were incredibly
: close, IsoDidact's stewardship of the Halo Areay, and the "Last time
: you asked me" bit, I think it's safe to assume that John carries
: within him the essence of the Forerunner. Therefore the rampant Spark at
: the end of Halo 3 would have no reservations about calling out his old
: friend.

I've wondered about this theory; as it seems to me a Forerunner might not be able to pass on an essence to a different species. On the other hand, they can bloody-well pass on their essences (along with a tonne of other fantastic technological wizardry), so cross-species might not be all that more difficult.

I feel like Guilty Spark's "Last time you asked me" statement was more due to partial rampancy. Poor confused little bugger. Although, him seeing the IsoDidact's essence in the Master Chief could explain why Guilty Spark assumed the Chief knew what the hell was going on. Heh- I could go back and forth with myself on this forever.

Whatever the case, it's one of my favorite moments in all of Halo. An enigma to whet the appetite for more stories! Years and years and years of stories! Whether or not it's been planned from the beginning, or ingeniously cobbled together over the years, I'm enjoying the direction Halo is going.

Why would you hesitate to do what you've already done?