Backtrack is a penetration testing distribution. Their latest/current version is version 5 which is based off ubuntu. They have provided ARM version which will work with devices such as N900 or generally any other ARM powered devices.

What Backtrack 5 ARM version doesn't provide is their kernel because every ARM device can be configured differently (in terms of hardware specifications). Therefore it is not possible to boot into Backtrack 5 ARM version without running it in the case of chroot, or with an advanced setup that either runs maemo's kernel or some other kernel that already works with N900 or whatever device you choose to boot.

The person who originally made Backtrack 5 work with ARM device was not using Nokia N900 but I believe an ARM powered smartphone running Android. If that's the case, that could be the reason as to why there's no kernel available with the image.