The little sphere test I just did I got no alpha channel w/ targa and one w/ tif. hmmm.
Didn't have the 32bit option for targa checked. That did it. I actually did get an alpha channel for both formats tho.

Are you on a mac or pc? I suppose it is possible the mac tiff format doesn't support alpha. This is on a pc.

No need to think outside the box....
    Just make it invisible.