again, for my custom install location I need to prepare:
- cleanly uninstall and remove cruft:
1$ dir=/Users/Developer.SnowLeopard
2$ sudo sh $dir/Library/uninstall-devtools
3$ sudo rm -r $dir/*
4$ sudo mv /Developer /Developer.deleteme
- then do the custom-location install and
- finally restore some hotfix softlinks:
1$ dir=/Users/Developer.SnowLeopard
2$ sudo ln -s $dir/Platforms /Developer/Platforms
3$ sudo ln -s $dir/SDKs /Developer/SDKs
4$ sudo ln -s $dir/Applications/Xcode.app /Developer/Applications/Xcode.app
Not removing the cruft will get you this quite terse error message:

XCode 3.2.4 install without prior cleaning