Sad but true. I can't say I was ever a huge fan of the character, although I liked What Greg Rucka did with the series, and the Joe Kelly (i think?) JLA books featured her very well. But that said, I definitely AM a big fan of Joss Whedon, so this is kind of depressing news. If you head over to Whedon's corner of the 'verse you can read the reasoning in his own words. But basically it says he wasn't coming to agreements with the studio over the script. He phrases it in terms of creative differences, and them simply not seeing eye to eye, and even though I know it happens all the time with films, it's still annoying.
Maybe Joss will use this opportunity to get onto some great new personal project, that finally, hopefully, actually gets a chance to succeed.