I don't think 8C148a is "final"; final builds numbers never seem to end in a letter (my phone is reporting "4.2.1 (8C148)"). I haven't debugged apps on it yet (I got bored waiting for it to extract symbols and went home).
4.2 GM was 8C134, so you're certainly running newer software, but something somewhere is getting the build number wrong.
But I digress.
What directories are in /Developer/Platforms/iPhoneOS.platform/DeviceSupport? If "4.2.1 (8C148a)" doesn't exist but "4.2.1" does, you might have luck doing something like this in Terminal:
cd /Developer/Platforms/iPhoneOS.platform/DeviceSupport
ln -s "4.2.1" "4.2.1 (8C134a)"
Equivalently, if "4.2.1 (8C148)" exists, do something like this instead:
ln -s "4.2.1 (8C148)" "4.2.1 (8C148a)"
You can generally use this trick to get Xcode to talk to a beta device without installing a beta SDK; this is useful if your company has some people running the beta (for testing purposes) and other people on the latest "final" (for release purposes).