If your Mac Mini had Snow Leopard originally installed, you could have installed Windows Vista into BootCamp and it would still have worked upon your upgrade to Lion. If it is worth the effort (and your Mac Mini originally worked with Snow Leopard), you could backup, downgrade, install Windows Vista and then upgrade to Lion and restore your backups.
As to BootCamp 4.0.3:
"3. Which versions of Windows work with Boot Camp 4.0?
You can use Microsoft Windows 7 Home Premium, Microsoft Windows 7 Professional, or Microsoft Windows 7 Ultimate with Boot Camp 4.0. Windows XP and Windows Vista are not supported with Boot Camp 4.0. "
http://support.apple.com/kb/HT4818#3
Otherwise, you will need a program such as Parallels 7 to run Microsoft Windows Vista. I have the mid-2011 Mac Mini, which came with Lion pre-installed, but I was able to create a Windows XP environment inside of Parallels 7:
Using the same installation of Parallels 7 you can install Snow Leopard (with Rosetta) into Parallels 7 and run that in Lion and have access to your PowerPC applications:
[click on either image to enlarge]
Full Snow Leopard installation instructions are here:
http://forums.macrumors.com/showthread.php?t=1365439