Tieflings are Outsider (Native), which means they're native to the Material Plane. ToM's Dark Creature template makes a creature extraplanar when not on the Plane of Shadow.
To me, this seems to imply that the creature is native to the Plane of Shadow, which implies they're not native to the Material Plane. Does anything actually happen to their Native subtype, though? Which is to say, are they treated like a non-Native Outsider for the purposes of eating/sleeping/raising?
A tiefling (Monster Manual 209–10) who possesses the dark template (Tome of Magic 161) and who is on the Plane of Shadow has the type and subtype outsider (native). The same dark tiefling when on any other plane has the type and subtypes outsider (extraplanar, native). "Unlike true outsiders, native outsiders need to eat and sleep" (MM 312), hence, regardless of what plane they're on, dark tieflings must normally eat and sleep.
The presence or absence of the subtype extraplanar (MM 309) doesn't alter or delete subtype native (312). A creature can totally be both, and it's not even that weird (see this question). The extraplanar subtype indicates only that the creature is currently not on its plane of origin. It's code for This creature belongs on a different plane and that's all. Be careful not ascribe power to the extraplanar subtype: while some game elements do interact with the extraplanar subtype (see below), most of the time the extraplanar subtype has zero impact.
To be clear, the dark template on Size and Type says that a dark creature's "[t]ype and size are unchanged. Dark creatures encountered away from the Plane of Shadow have the extraplanar subtype" (Tome of Magic 161). When the dark template's applied, it changes a creature's plane of origin from the Material Plane to the Plane of Shadow, but the creature is, with respect to size and type, otherwise unchanged.
This means that, for example, a dark tiefling on the Plane of Shadow doesn't have the subtype extraplanar, but the same dark tiefling does have the subtype extraplanar on the Material Plane, the Abyss, or any other plane.
Note that a creature that possesses the extraplanar subtype can be the subject of the 5th-level Sor/Wiz spell dismissal [conj] (Player's Handbook 222) et al., for instance. If the caster succeeds in expelling the extraplanar creature, it returns to its plane of origin. Thus, a normal tiefling (if there is such a thing) residing on the Material Plane is not vulnerable to being sent by a dismissal spell to the Nine Hells (or wherever Great-grandma Hsidneif was from), but a dark tiefling residing on the Material Plane will be sent by a banishment spell to the Plane of Shadow.
First you have to understand that being native to a plane and having the Native subtype are two different things. Every creature is native to a plane, but only Outsiders native to the Material Plane have the Native subtype.
Native subtype, Monster Manual p.312:
A subtype applied only to outsiders. [...] Creatures with this subtype are native to the Material Plane [...]
Normal tieflings fall in the latter category, they are Outsiders native to the Material Plane, that's why they have the Native subtype.
But a dark tiefling (that is, one with the Dark Creature template from the Tome of Magic p.158) is different.
Tome of Magic, about Dark Creatures, p.158:
Other creatures native to the Plane of Shadow exist [...].
Tome of Magic, about creating Dark Creatures, p.161:
[...] Dark creatures encountered away from the Plane of Shadow have the extraplanar subtype.
Extraplanar subtype, Monster Manual p.309:
A subtype applied to any creature when it is on a plane other than its native plane. [...]
From these quotes it's clear that Dark Creatures are native to the Plane of Shadow, unless some other template or effect applied later changes that.
Therefore, applying the Dark Creature template to a Tiefling must mean that it loses the Native subtype, because being native to the Plane of Shadow and having the Native subtype is not possible.
Ergo, dark tieflings don't need to eat or sleep since they're normal outsiders.
