I thought you'd never ask.
Yes and No.
Dark Eldar like all humans are free moral agents, they can stop being evil psychos but it's very hard. The normal event means that if they became "good" they would fade away and die due to their ailment (Soul Decay). The Eldar farseers tried to combat this, initial subjects would still fade away, their soul decay simply being delayed. The only effective way, as of now, is to merge the soul with an aspect shine, immortalising the soul as an exarch - this takes a lot of effort however and takes over a millennium for the thirst to truly die. One of the Striking Scorpions on Craftworld Alaitoc is a Dark Eldar (ex-Incubi) - he is the only explicitly declared Dark Eldar/Eldar.
Thought I hasn't been offically said I think it safe to say that most Harlequins are former Dark Eldar. Harlequins are units for the two major Eldar factions in the lore and recruit their numbers from both races. The reason I think Harlequins are mainly Dark Eldar is because only a self hating/guilty/desperate Dark Eldar would risk the Harlequin ritual. The ritual frees you from Slaanesh in every way - unlike Eldar who go still go to hell after they die/have their soul crystals crushed, Harlequins are forever free. Eldar are relatively free, Dark Eldar, not so much. The risk of the Harlequin ritual is a) death then hell and b) becoming soulless and shunned by all living things.
The above only works with "Trueborn" Dark Eldar, Dark Eldar born from the womb. Eldar reproduction can be replicated by artificial means. Naturally, Eldar zygots grow in terms. In humans you fertilise the egg once and wait the 9 months, in Eldar continued intercourse is required, I believe four or three fertilisations must occur during the pregnancy - for this reason Eldar have allele sets not allele pairs. Theoretically it may be possible for an Eldar to have two or three biological fathers since we don't know if it has to be same father every time. The Eldar term however takes a very long time to finish (centuries?) to finish so Dark Eldar have got past this, only 1 in 16 Dark Eldar is True born the rest are literally feed by experience by Haemonculli in vats. If there is enough pain and suffering afflicted near by the Eldar can grow in under 3 days. Even in dog eat dog violent world of Dark Eldar part of the Webway the Dark Eldar population is actually growing.
I don't know if these Eldar, born from pain cyrstals, can become reformed.
-- This idea featured in Gav Thorpe's "Path of the Warrior". He wrote the 3rd edition Eldar Codex and created most of the detail about Craftworld Eldar.
[img=500x500]http://suptg.thisisnotatrueending.com/archive/14903944/images/1305229948969.jpg[/img]<-- Pic related it's a Dark Trolldar