Here in Taiwan at elementary school they seem to come with a set of lines for writing practice on one side and blank on the other.
I have used them for students to practice learning to write new letters of the alphabet before putting them into their exercise books. It makes their books neater and they gain in confidence.
They are also good for pair work. An example is answering jumbled words or sentences and being quick to display their response by raising the board above their heads.
There are many uses for this simple tool.
Thanks for the other sharings also.
Pasted today at http://classroom201x.wordpress.com/2010/07/06/back-to-basics-mini-whiteboards/#comment-112
You will find many more ideas for using mini whiteboards at this site and the included links.