I've set up the standard "User Login" block to the "header" region of my theme. I clicked on "configure" and selected "Link" under "Block Type". Now the link to sign in appears as "Log In/Register". How do I change this text? I have also LoginToboggan enabled.
Instead of throwing a string override directly into settings.php file, use the excellent String Overrides module. It can help you to manage the string overrides for your login link, as well as possible future UI text changes. All this without hacking core files.
You could use a template, probably named something like 'user-login.tpl.php'
For more information on 'working with template suggestions' have a look at http://drupal.org/node/223440