In both cases, I mix the same image with plain white background.

Here are some more examples. As you can see in the details, it seems that in sequencer some black color, or dark, is added to the alpha values.

Okay, I've downloaded the blend file, but could you also upload some of the test images that you used to test the bug out so that I could look at it myself?

The following is the file and the image I generated with some alpha values.

Below is a test I made rendering on Sequencer and on Compositor, using the same effect "Alpha Over".

Now even though I'm running on Windows 7 Professional x64-bit, this has happened to me too.

Here's what it looks like for me in both the compositor and the sequencer:

Here's a scene of what this looks like for me in the compositor:

...and here's the result:

Here's a scene of what this looks like for me in the sequencer:

...and here's the result:

I've provided some files for you look at if you want to which include the image that you've uploaded:

Now, even though it works for me too, it doesn't seem to be a bug at all. It's probably an issue with the video sequence editor. So, while working in Blender, when rendering an alpha over image, you'll have to compare and contrast of what image looks best whether it's in the compositor or the sequencer. Maybe...... just maybe....... this issue will probably get fixed in a later version or svn build of Blender in the future. We'll just have to see.

Thanks for reproducing the issue.

The last thing I want to see is that kind of war "it is not a bug, it is a feature". As you mentioned, it is an issue with the sequencer, so it is a bug, and I think it should be treated properly. This is an option/effect without the expected behavior.