When you reply to a message, Apple Mail quotes the text of the message to which you're replying after a line like "On May 4 2005, at 5:16 PM, John Doe wrote:". For some reason, though, Mail puts a blank line after that line, separating it from the quoted text, and skips putting one before it, so it's right under the cursor and ends up run in with your typed message. It's weird and ugly!
To fix this from the Terminal, type this:
sudo vi /System/Library/Frameworks/Message.framework/Versions/\
B/Resources/English.lproj/Delayed.strings
Change the line that reads:
"REPLY_ATTRIBUTION" = "On %1$@, at %2$@, %3$@ wrote:nn";
to this:
"REPLY_ATTRIBUTION" = "nOn %1$@, at %2$@, %3$@ wrote:";
Then you'll probably have to restart to clear the system's cache and have the changes take effect.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050505001300161