I don't get why people are upset that IE9 may not run on XP. Did they think that running XP is a perpetual entitlement to be able to run all future Microsoft software? What can IE9 do that's so important that IE8 or any of the competing browsers can't do?
I don't think it's people running XP who are disappointed. If this news means much to you you're probably tech-savvy enough to have installed a more capable browser.
Developers must be disappointed because this means the millions of non-tech users running XP (and not interested/capable in upgrading to 7) won't be automatically upgraded to a browser that can run websites built on modern technologies. Consumer-oriented HTML5-powered webapps will continue to require kludges or hacks or concessions to work for many users, and will keep requiring these until a critical mass of grandpa- or aunties-owned computer machines break and grandpas and aunties go to the Best Buy and get new ones.