When you upgrade from 10.0.4 to 10.1 using the CD, there are a few things that may occur:
1) Your Internet Explorer and Late Breaking News icons will turn into question marks (meaning they are now referencing obsolete pointers). You can just drag these off to the desktop to erase them, and drag the new Internet Explorer 5.1 icon from Applications onto the dock. <confirmed by Apple Support>
2) The Preferences for Address Book becomes grayed-out and inaccessible. Moving/deleting the old plist and restarting the application to create a new one doesn't solve this problem. <confirmed by Apple Support>
3) I have been unable to compile anything since the upgrade (compiling main(){return(0);} errors with "/usr/bin/ld: /usr/lib/libSystem.dylib load command 6 unknown cmd field") <under investigation by Apple Support>
Note: at this moment the new Developer Tools CD will only be available to people who pay their $20 or buy a 10.1 package after Saturday {i.e., not for free giveaway in stores} and is currently unavailable for download -- I only had the old Developer Tools installed before the upgrade.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010927213956100