When trying to install the Command Line Tools from the Downloads tab in the Xcode 4.3 preferences we got the error “invalid checksum”. We were able to fix that problem by clearing the dns cache on our system.
This is the Terminal command to clear the dns cache (OSX Lion):
dscacheutil -flushcache
Restart XCode after running this command, it may not fix the problem with the checksum until you do.
Found via http://stackoverflow.com.