When we were building the new Basecamp, we wanted the foundation to be built on clean, modern underpinnings to take advantage of all the new wonderful features of HTML5. That meant we have to drop support for older browsers, like IE8, that have little or poor support for the HTML5 technology we are using to make Basecamp awesome for everyone.
But, have no fear! We realize that a lot of people are stuck with IE8 (sometimes even IE7 or, yikes, 6), so we made sure that Chrome Frame works with Basecamp. Chrome Frame is available for IE 6-9 on Windows machines and can usually be installed without admin access.