All very well when you're sitting at home but have you ever worked in a large corporation? Most PCs aren't powerful enough to run a second virtual OS instance and, even if they could, maintaining, patching and securing a second OS on every PC effectively doubles the admin overhead of the network - not to mention the licensing cost of doubling the number of antivirus seats you have etc. Virtual XP mode is only suitable for home users and for very specific cases in larger organisations. For large-scale rollout another solution is needed. If you want to stick with the virtual XP based solution but have it manageable, Microsoft have MED-V which will happily run an seamlessly instanced IE6 (as virtual XP mode does) but is clever enough to automatically switch between the native IE8 browser and the virtual IE6 browser based on which URLs you are visiting. MED-V still suffers from the increased hardware requirements of running a second virtualized OS on a client PC. Other alternatives are to deploy IE6 using Citrix XenApp running on a Windows 2003-based server but this also suffers from the same issues or VMware have just announced full support for IE6 running under ThinApp which is probably the least-worst option for most organisations if it weren't for the licensing angle.
Microsoft have a huge number of tools and information on performing compatibility testing prior to a Win7 rollout and anyone considering it I would highly recommend looking into the (free) Application Compatibility Toolkit (ACT). For more thorough appcompat testing, look at the toolset provided by App-DNA which is fantastic.
