It kind of depends. If the attachment is something the iPad can view (for example, picture or pdf) then you can tap on the attachment and view it directly from the mail message. But because there is no direct file system you can't save an attachment to a folder like you would on a Mac or PC client.
This page explains some of the basics: http://www.my-iguru.com/ipad/ipad-hints-tips/ipad-viewing-saving-email-attachmen ts.php
There are some apps you can download to expand the number of document types you can open in an attatchment on an iPad.