I've been using a similar filter targeted at IE6 for over two years now. First PNGHack by Aaron Porter and as of last year, IE PNG Fix by Angus Turnbull.

I prefer this last one to what was just linked above. It can handle background images without a problem, but the way I have it set up you will specify the "behavior" for specific background selectors instead of everything on the page. Applying to everything would be overkill in most cases.

Since IE (any version) will always require a separate style sheet to make up for box properties and other crap MS just gets wrong, it's the perfect place to also include the behaviors without letting other browsers see them. I now keep a general IE css file and then an IE6-specific css file as well.

Much of what's broken with IE 6 is also broken in IE7 unfortunately. The smartest thing MS could possibly do would be to toss their crusty old and hacked-up Mosaic-based engine in favor of Webkit. I won't hold my breath though.

Matt, if you need any implementation details for this you can contact me via email. I'm just working on some new web layouts right now and have again made extensive use of alpha and IE fixes of all sorts.