hey, i had this trouble too, simple solution. when your make the decal, either use pure black, or pure green, as the BG, i prefer green as black crops up naturally in images / text. then in dxtbmp, in the alpha menu, select "create alpha channel (green)" or (black). and that makes the background (actually anthing pure green (or black) transparent, but only when saved as DXT1. NOT dxt3. Any probs, post back or PM me.

good stuff, just to confuse things, alpha is used in defferent ways depending on image format, and what the model is set up to do! in some, even using DXT3 and alpha, the plane becomes transparent, with the same gradient of 'see-through-ness' as you'd get with reflectivity. lol anyways, you dont need to bother about that, but remember it in case your aicraft suddenly goes invisable even using dxt3 or 32bit textures  8)