I recommend that you use a 24 bit PNG like Wildhoney says. I've been working alot with 24bit alpha PNG's lately and they are incredibly good to use. Except for IE6, though which is pretty easy to fix. (iepngfix)

What I do with all my png's is run them through pngcrush and remove all colour information. Pngcrush will not only optimize the file, but remove irrelevant color information from the file. Sometimes it can save alot of filesize.

Also, if you don't remove the color information, Internet Explorer (on windows anyways) tends to look at the color information and alter the colors accordingly. Hence sometimes when using png's as a background, the html hex color code won't be the same as in the PNG (in IE) because of the color information.

If you need any help with this I'll gladly help. I could take the original wine picture and give you back a 24 bit transparent png image, crushed, looking the same in all browsers and blending perfectly into the background.

I've actually been thinking about writing an article on PNG usage with CSS for TalkPHP (if anyone is interested). (also tiny photoshop tips for making it blend in)

And supplying a little library of images for navigation menus, hovers, backgrounds etc. The possibilities are almost endless.