To further the difficulty mentioned in Chrystal's comment, you cannot gain adverse possession on land where the title has reverted back to the city/county or state for non payment of taxes.

While squatting in "abandoned" houses may [[and I cannot stress the "MAY") have some beneficial effects to a neighborhood - allowing this type of occupancy can bring a whole host of issues for landowners who have an investment [[emotional, moral, financial, etc.) in their property and find someone squatting.

Just my two bits.