Apple (secretly?) posted a tech note on how to troubleshoot installing BC 2.1
"Errors occurred while installing the updates. If the problem persists, choose Tools > Download Only and try installing manually" may appear during installation.
"Failed to extract matching transform" messages may appear in the log when attempting to install.
Products Affected
Boot Camp, Microsoft Windows XP, Microsoft Windows Vista
In Windows, start a command prompt (cmd.exe, sometimes referred to as a command shell). Use Administrative rights if running Windows Vista.
Type one the following command lines in the command shell:
For Windows XP, type: msiexec /update <path to the .msp file>\BCUpdateXP.msp
For Windows Vista 32 bit, type: msiexec /update <path to the .msp file>\BCUpdateVista32.msp
For Windows Vista 64 bit, type: msiexec /update <path to the .msp file>\BCUpdateVista64.msp
Click Next
Accept the terms of the license agreement if you agree, then click Next
Click Repair/Update
Your Boot Camp drivers will be updated to version 2.1. This may take several minutes.
Click Finish to exit the installer and restart your computer
http://support.apple.com/kb/HT1845?viewlocale=en_US
What I want is to see a link to this or other articles on the BC 2.1 update page, something like "if you have trouble with this update, please see this article."