Strange... in my testing, the Mendix username is case-in-sensitive. The password is case-sensitive, but not the username.
Do you have LDAP authentication, so the case-sensitivity is coming from LDAP and not Mendix?
Edit: If you are using login.html as your login page, you could add a javascript onChange action to the form as follows:
<input type="textfield" name="username" class="formInput" onchange="javascript:this.value = this.value.toLowerCase();"/>
Edit 2: If you are using the Login Form widget, you can make a similar change. Find the LoginSystem.mpk file in your project widgets directory, rename it to a .zip extension then unzip the contents. Locate the LoginForm.html file in the templates directory and edit it to add the same javascript action:
<input type="text" class="${baseClass}_usernameInput" dojoattachpoint="usernameInput" onchange="javascript:this.value = this.value.toLowerCase();"/>
Zip up the widget again and change the extension back to .mpk
Changing an HTML page or a widget is much safer than editing mendix.js file IMHO