If you use photoshop there is an even easier way then using the eraser. Select the layer with your text, go to the select menu then select couloir range, then just select the black and then you can layer via cut. Delete the new layer an you are left with just your text. If you then need an alpha channel which some program's need, just right click the layer and select pixels. Then go to channels and create a new layer. Then if you press shift f5 and choose full with white you create a perfect alpha channel. You can then just say this as a png or tiff and you will have your text with an alpha channel built in.