Remember to put the rock back carefully and in as close to the the exact same spot as you can so as to not hurt the little creatures living underneath. Where our family lives we don't really have any dangerous creatures to watch for that I can think of, but it's good to be careful. Be sure to be cautious and watchful for poisonous animals such as scorpions if you live in an area that has such creatures. They recommend wearing gloves.
This will be our first year participating in International Rock Flipping Day, but we've looked under things in our yard many times to see what we could find. This should be fun! :-)
If you want to see what it's like, here's a link to the page on Wanderin' Weeta's website with last year's entries: IRFD 2009.