Security question: "Text file busy" overwriting executables but not shared libraries?

[Rob Landley]

Anybody want to venture an opinion why overwriting executable files that are 
currently in use gives you a "text file busy" error, but overwriting shared 
libraries that are in use apparently works just fine (modulo a core dump if 
you aren't subtle about your run-time patching)?

Permissions are still enforced, but it seems to me somebody who cracks root 
on a system could potentially modify the behavior of important system daemons 
without changing their process ID numbers.

Did I miss something somewhere?

Rob