I have been asked to do a 11x15 sheet cake with a plaid design going up the sides and over the top. She also wants me to write on it. I am concerned about keeping the lines straight. I do have an airbrush but haven't used it much. Any suggestions??

You can use MMF and cut strips of it. I did a cake like this . It is in my photos. Here is the link:
http://www.cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=allby&uname=luv2cake&cat=0&pos=20

It is sort of plaid. But it is an easy way to keep your lines strait.

or a two-color basketweave maybe?

Here is one I did in buttercream. You have to be really careful when doing the horizontal lines. Its easy to get off. Its time consuming but not really that hard. Just an idea.

I did one using fondant: