A trick I use to get rid of some artifacts caused by displaying scaled or rotated transparent textures in my 2D games is as follows.

[pascal] glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDisable(GL_ALPHA_TEST);[/pascal]

This is just so much better than playing with Alpha Test. Especially when you are using a pre-rendering method of creating sprites.