It is also mentioned on the Arduino frontpage, but I thought it was good to mention it here too.
Libelium is having an Arduino Contest for 2008. See here for more information: http://www.libelium.com/tienda/catalog/contest.php
There are 3 categories where you can participate and the winner of each category will get one GPRS/GPS Module for Arduino (exclusive and limited edition)
Gentlemen, start your engines :)