Do mormons have a doctrine of afterlife preaching and repentence? I swear I read somewhere some years ago that Mormons have a developed doctrine of saints in the afterlife descending to Hell in order to preach the gospel to those who are stuck there and give them the opportunity to accept Christ and repent.

I find this interesting and appealing because it would neatly solve the age-old dilemma of the dude in the jungle who went his whole life without ever hearing the Gospel. He ends up in Hell, but the friendly mormon missionaries take a holiday from heaven to come and hang out with the damned where they "preach to the spirits in prison". This jungledude then has the opportunity to accept the gospel. Presumably there is no time limit set on the offer of salvation and therefore it is safe to assume that everyone will eventually end up responding positively to the gospel (either here on earth or after death in Hell).

My question is, is any of what I just said accurate? Do Mormons actually believe anything similar to this?

(I note that what I just described is slightly similar to the "Holy Saturday"/"Harrowing of Hades" tradition in the Catholic/Orthodox churches: this is where Jesus "descended to Hell" and busted out of hades/sheol all the righteous people from the old testament times)

(Can someone please add the tag "harrowing-of-hell" or "holy-saturday" (these are basically synonymous))
Not only is this belief held by members of The Church of Jesus Christ of Latter-day Saints, it is one of the core tenets of their faith as outlined in the Four-fold Mission of the Church. 

Mormons believe that after physical death the spirit is separated from the body and enters the Spirit World. The spirit is then places in Spirit Prison or Spirit Paradise, based on the person's words and deeds during their life. A primary reference for this belief can be found in Alma:40 in the Book of Mormon.

Mormons believe that when Christ was in the Spirit World prior to his Resurrection he organized a ministry among the righteous dead assembled in the Spirit Paradise and bridged the gulf between Spirit Paradise and Spirit Prison. This can be found in Doctrine and Covenants Section 138.

Hand in hand with the belief of the ministry to the dead is the doctrine of Baptism for the Dead. Members of the LDS church observe the practice of Baptism for the Dead in which they are baptizes as proxy for their ancestors. This gives those in Spirit Prison who accept the gospel posthumously a chance to receive the necessary saving ordinances to enter into Spirit Paradise. Baptism for the Dead is only done within Temples and is seen as a very sacred ordinance. More about the Baptism for the Dead can be found in Doctrine and Covenants Section 127 and 128.