On Microsoft's Live single sign on system they seem to employ some kind of trickery to prevent browsers from wanting to save passwords. (Or at the very least Firefox behaves as such). I have the Password Saver bookmarklet that manipulates autocomplete on forms/fields to not dissuade the browser from asking me to save my credentials however this doesn't seem to have any impact with Live.
They're probably changing the field names in the HTML code each time someone visits the page. When Mozilla's, Opera's, or other browsers try to save the "password" type input field data, they also save the field name as defined in the "name" value of the relevant HTML "input" tag.
If the "name" field (which the user doesn't see) always includes a changing (unique or random?) number, for example, then this will throw these systems off since they'll be expecting the same previously stored input field name (and not finding it) upon return visits.
This is a technique that I find to be rarely used on web sites where the webmaster considers these "remember password" features in web browsers to be a serious security risk.
I found the following code by selecting the Windows Live ID input field, right-clicking it, and selecting View Selection Source under Firefox:
<input class="cssTextInput" autocomplete="off" maxlength="113" id="i0116" name="login" type="text">