Ask HN: Dealing with Constant Browser Upgrades

There have been small discussions in many threads about the "brave new world" of web development caused by the rapid release cycles of Chrome and Firefox. I'm currently working on a web app to be used a highly regulated industry that typically requires buckets of QC. My group is struggling with both settling on a list of accepted browsers and dealing with testing those browsers. How are others dealing with this? I've suggested that we define a "current" list of browsers (say, FF 3 to 7, and Chrome 14), test against those, and let our user base find bugs in later versions.