I believe you can use the
Select -> Color Range... tool to achieve this affect. The color range selection tool is a pretty handy feature that lets you click a color from a gray-scaled version of your image, then use a "fuzziness" slider to adjust the range. You can also preview the selection in the full image. This creates a normal photoshop mask when you choose OK.
Once you have your baseline selection, you can then further adjust the mask using all of the normal masking tools in Photoshop. This is probably the most ideal way to do what you need to do (based on your previous question, your trying to isolate a bottle?) The Color Range selection will give you an almost-ideal mask with transparency in tact...and you can further adjust that to fix areas that were selected that should not have been, or that were excluded and should have been selected. Once you have fine-tuned your mask, simply hit
Delete, and watch the magic occur. Since the mask was a full alpha mask, the deletion will remove varying degrees of "opacity" based on how heavily an area matched the base color you selected initially.
Since your background is white, selecting the white color and adjusting fuzziness to appropriately "fill in" the parts of white that show through your bottles should give you a pretty effective mask in about 30 seconds flat. A little adjusting might take a couple minutes, but the end result should be better than the Gimp "Color to Alpha" plugin (at least, based on the few examples I've seen...it seems to be a bit over-zealous in its application of transparency.)
Generally speaking, a "full green" or "full blue" background might be better for this kind of thing. White is a blend of all colors, and that can make it more difficult to isolate specific objects from an "all-color" background. Thats usually why a lot of special effects sequences shot for movies are filmed against a green backgrop...its easier to filter out during post-process compositing (its a single "primary" color in the RGB model, rather than a blend of all three colors. This is called Chroma Key filtering.) It should be noted that you want to use a chroma key that is complementary to the color(s) of the object being shot. For example, if you are shooting a green bottle, it might actually be best to use a red background, as using a green background would make it very difficult to exclude a green background without losing the bottle along with it. If you have yellow, orange, red or brown bottles, a blue background may be best. The general idea, though, is to use a background that is complementary (think opposite color on the color wheel) to the foreground objects being shot.