Hi,

[Magento Version 1.4.1.1]
Magento provides a small login form that the user could use without navigating away from his current page, we can see this in customer.xml layout:

<!--
Load this update on every page when customer is logged in
-->

    <customer_logged_in>
        <reference name="top.links">
            <action method="addLink" translate="label title" module="customer"><label>Log Out</label><url helper="customer/getLogoutUrl"/><title>Log Out</title><prepare/><urlParams/><position>100</position></action>
        </reference>
    </customer_logged_in>

<!--
Load this update on every page when customer is logged out
-->

    <customer_logged_out>
        <reference name="right">
            <block type="customer/form_login" name="customer_form_mini_login" before="-" template="customer/form/mini.login.phtml"/>
        </reference>
        <reference name="top.links">
            <action method="addLink" translate="label title" module="customer"><label>Log In</label><url helper="customer/getLoginUrl"/><title>Log In</title><prepare/><urlParams/><position>100</position></action>
        </reference>
        <remove name="wishlist_sidebar"></remove>
        <remove name="reorder"></remove>
    </customer_logged_out>

We are using the mini login form on the homepage, and we needed a way to display a different message depending on the user login status, the default behavior was to hide the whole form (we can see that the block is missing from the customer_logged_in login tag).

We change the mini form template to fit our needs as follows:
/app/design/frontend/default/default/template/customer/form/mini.login.phtml

<?php $rLoggedIn = $this->helper('customer')->isLoggedIn(); ?>

<div class="block block-login">
    <div class="block-title">
        <strong><span><?php echo $rLoggedIn ? 'WELCOMe' : $this->__('Login') ?></span></strong>
    </div>
    <?php if ($rLoggedIn) { ?>
        <p class="welcome-msg" style="margin: 15px;">  
            Welcome back <strong><?php echo Mage::getSingleton('customer/session')->getCustomer()->getFirstname(); ?></strong>, <br /><br />
            Browse to your <a href="<?php echo $this->getUrl('customer/account') ?>">Dashboard</a>
        </p>
    <?php } else { ?>
        <form action="<?php echo $this->getPostActionUrl() ?>" method="post">
            <div class="block-content">
                <label for="mini-login"><?php echo $this->__('Email:') ?></label><input type="text" name="login[username]" id="mini-login" class="input-text" />
                <label for="mini-password"><?php echo $this->__('Password:') ?></label><input type="password" name="login[password]" id="mini-password" class="input-text" />
                <div class="actions">
                    <button type="submit" class="button"><span><span><?php echo $this->__('Login') ?></span></span></button>
                </div>
            </div>
        </form>        
    <?php } ?>
</div>

Hope that helps.