Web designers have clamored for it for years, since they saw what you could do with PNG images’ multi-level transparency (now available in every major browser except Internet Explorer, and several minor ones). It’s been speculated on for months, and rumored for several weeks.
And now, the wait is over. Microsoft has confirmed a few details about IE7 beta 1:
Support the alpha channel in PNG images. We’ve actually had this on our radar for a long time, and have had it supported in the code for a while now. We have certainly heard the clear feedback from the web design community that per-pixel alpha is a really important feature.