Normally, when you are editing text, Command-Right Arrow will move you to the end of a line of text, and Command-Shift-Right Arrow will select to the end of a line of text.
However, in Safari, Command-Shift-Right Arrow will select the next tab (or do nothing if you only have one tab open). To work around this, use Control-Shift-Right Arrow instead. Similarly for Command-Shift-Left Arrow, use Control-Shift-Left Arrow.
I don't know how Command-Shift-Arrow works if you have tabs disabled, but everybody uses tabs anyway, right?
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031212153631177