Humble,
Rip is right, they can be killed. The Cannon Hawg still does a decent job of it (while not as good as before this release, but I like it better now anyway for gameplay issues). The Tiffy does a nice job as well, just not as good as the Cannon Hawg. Even a Spitfire will get a kill if you land the shots just right.
To kill the tank you have to hit them from the rear in the engine compartment. Hit them there and you will get a kill on 2 passes for sure. If you are attacking them from the rear and don't get the kill, then you are missing the engine area. You can also knock out the turret or driver with an approach from any other angle, you just won't get that nice explosion.
A smart tank driver (<S> Rip) will try not to give that aspect shot, or will try to use terrain (a hill) to back his arse into so that you can't get the shot that easy.
Rockets and bombs will kill a tank as well.
Last week, a squadmate and myself, made formation strafing runs on some tanks in spits, and then later in Cannon Hawgs. It proved to be a very effective air-to-ground way of dealing with those tankers. It also highlighted for me how great it would be to have other strat targets to strafe and bomb/rocket such as trains, rail depots, bridges. That formation strafing is alot of fun.
Cobra