you could insert a link in the product description with URL "http://<your_domain>/login.php" and embed this in the text you want: e.g. if you use the plain text editor in your product description, the inserted code could be:
Code:
<a href="http://<your_domain>/login.php">Login to view Special Dealer Pricing</a>
where you replace "<your_domain>" with your domain. If x-cart is installed under a directory then you use:
Code:
<a href="http://<your_domain>/xcart_root_dir/login.php">Login to view Special Dealer Pricing</a>
where "xcart_root_dir" is the name of the directory your x-cart is installed under.