There were a couple people who have tried starting seeds inside only to have their cats knock them over or eat them! While my dog wants nothing to do with our plants, we do turn the heat down significantly while we're gone so our seeds usually have a tough time getting started. Here's a recycled greenhouse idea that I think might help remedy both problems.
JoeysPlanting Flickr stream has some great ideas on sowing seeds and utilizes a great rotisserie chicken container.
You could either place your potting medium directly into the container (though drainage might be an issue unless you make holes and use a drainage plate) or place smaller containers (part of an egg crate?) inside your new greenhouse.
There is also an Instructables that details the steps to a great mini recycled greenhouse - here are some photos below: