Do you mean in the iPad Forum site? If you're using Safari and the ipadforums.net website, then there's a drop down menu in the Message or Quick Reply box with a range of fonts available there.
If you want to use fonts in, say, an email message then, although the iPad's Mail app can display a wide range of fonts, there's no way to generate a message from within Mail that uses anything other than the default font. However, there is a work around. Since Mail can display HTML coded messages, I use a third-party HTML editor and compose my message in there. Once it's finished, I simply copy-and-paste it into the Mail message box.
Tim