If you upgrade to Lion from Snow Leopard + Win7 bootcamp, you might find that you can't download new bootcamp 4.0 from Bootcamp Assistant. The way I go around it is by downloading it from Apple's CDN:
- Download Bootcamp 4.0 Drivers, here's a link on some Apple CDN for the .pkg file containing the drivers (603MB): http://bit.fosk.it/bootcamp40
- Open this file with 7Zip in Windows, and go through a few layers to extract BootCampESD.pkg\Payload\Payload~\.\Library\Application Support\BootCamp\WindowsSupport.dmg\0.Apple_ISO
- Rename the file from 0.Apple_ISO to BootCamp4.iso
- Extract the iso with 7Zip to a folder.
- Open command prompt and navigate to the extracted folder, for example:
- From the prompt in that folder, run
msiexec /i BootCamp64.msi