Short version: If you're sending usernames and passwords over the wire in plaintext, you've already opened a huge security hole.
Long version: Browsers will cache based on URI, so if you're sending user/pass as GET variables, then yes it will cache and yes it is a security risk. However, even if the browser didn't cache this, you're still doing something wrong. A third party need only look at the HTTP header to see what the user/pass is.
If you are sending this as POST, it is a bit harder to find the username/password. The browser will not cache the request as the URL is always the same. However, it is still possible to read the content of the request and find the user/pass.
To be the most secure, use HTTPS and pass the values via POST. The entire HTTP request is encrypted, including the headers. However, the browser will still cache the URL, so using GET variables is still a bad idea.
Example from the jQuery documentation on using POST with ajax:
$.ajax({
type: "POST",
url: "some.php",
data: { name: "John", location: "Boston" }
}).done(function( msg ) {
alert( "Data Saved: " + msg );
});