Weird aliasing problem on text border

[synhaptein]

Basically I put a text, set a background black with an opacity of 0.3 and add it to an image with a white background.

Anyone have an idea why I got those aliasing problems (not sure if it's call like that!?) on the border of the letters?

I don't know enough about video/image processing to understand what's really going on here, but from what I understand, when the text is merged with the background, the alpha is ignored?