An interesting note for people using either Windows XP or Windows Vista with Boot Camp is that you will not be able to install the latest version of Boot Camp (version 4) on your system. In a recent knowledgebase article, Apple outlines the system requirements for Boot Camp 4, part of which is the requirement for "an authentic, single, full-installation, 32-bit or 64-bit Microsoft Windows 7 Home Premium, Professional, or Ultimate disc."
While Boot Camp 4.0 will run in either Snow Leopard or Lion, it is a requirement for setting up new Windows installations in Lion even though existing Boot Camp 3 installations will continue to work in Lion if you upgrade your systems. What this means is that if you have an existing Windows XP or Vista installation when you upgrade to Lion then Boot Camp will still work, but you will not be able to upgrade it to version 4 or later. Additionally, since Boot Camp 4 is the default version for Lion and is required to set up new Windows installations, if you decide to set up a dual-boot environment for Windows from a Lion installation then you will need to have Windows 7 or later, as your older XP or Vista discs will not work.
This should ultimately not be much of an issue for people installing Lion on their current systems since current Boot Camp drivers in Windows XP and Vista should work indefinitely; however, if you are migrating to a new system or wish to wipe your drive clean and set up Lion and your Boot Camp installation again, then you may run into difficulties. For people attempting to do this, there are some options to get around Lion's requirements and keep their older Boot Camp installations.
Downgrade then upgrade
If your system shipped with Snow Leopard or earlier, you can install Snow Leopard and then set up your Boot Camp environment before upgrading to Lion. If you have proper Time Machine backups then you can do this with minimal impact on your workflow by either migrating your data from the Time Machine backup, or simply restoring the backup of your choice.
Clone Windows installation
If you wish to keep your current Windows installation intact instead of having to reinstall it, then you can use a tool like WinClone to make a bootable backup of your Windows partition, and then restore it to your boot drive. One option here is to use the Boot Camp setup assistant in Lion to create the Windows partition, and then instead of installing Windows through the assistant you can use WinClone to restore your installation to the new partition.
Cloning in this manner is likely the only way to get a Windows XP or Vista installation onto a new Mac that shipped with OS X Lion; however, do keep in mind that the older Boot Camp drivers may not be fully compatible with the newer Mac hardware.
The last option is one I recommend unless you need the optimal speed from your Windows installation, and that is to virtualize your Windows installation instead of running it in a dual-boot environment. Using tools such as Parallels Desktop or VMWare Fusion, you can migrate your Boot Camp installation to a virtual machine file to run within OS X, and have Windows applications appear side-by-side with your OS X programs.