If you place a Redstone Torch anywhere next to TNT, it will not prime, but if you place another Redstone Torch next to it, and destroy either of the Torches, then it triggers.

It also triggers if you send a Redstone Update to the Torch which is meant to ignite it, by for example placing another Redstone Torch next to it.

Also, if the Torch is already placed and you place TNT next to it, it triggers as intended.

Lever works fine.

NOTE: May be somehow related to MC-266.

Screenshots:
2013-01-25_22.46.53 - TNT not triggering.
2013-01-25_22.44.40 - TNT triggered as soon as the Torch diagonal of TNT got placed.
2013-01-25_22.44.53 - TNT still not triggering.
2013-01-25_22.44.54 - TNT triggers as soon as that Redstone Torch got removed.