is it possible to get a room for a single night only? i checked at least 10 places and everywhere claims to have a 2 night minimum on their websites.

Since July/August are peak months for RI travel many places will require 2 night stay. Where were you hoping to stay (on or off island)?and is it on a weekend?
Edited: 10:44 pm, January 27, 2013on the island for saturday night. we'd love to stay friday but i dont think we can make it from the city to one of the ferrys without missing a substanial chunk of work on friday afternoon. any advice here? our plan now is to do friday night in MTK and then take the ferry from there first thing saturday.
Unfortunately that time of year on a weekend it will be nearly impossible to find a one night stay, my guess would be the only way to get a one night stay is wait til last minute then call a bunch of hotels and see if they have a vacancy (people cancel all the time and the hotel would rather ril tat room for one night than not at all). Keep in mind later ferries normally run in July/ Aug since so many people visit the island you may want to check with the ferry company you'll be using and see if that's the case for them.
I agree with previous posts, because it's peak season, rooms are generally 2 night minimum (3 holiday weekends) but you can get a room for 1 night if there is availability on that day. I have had a lot of success doing that but you always take a chance because you have to wait until the last minute. That being said, the ferry schedule increases the number of boats during the summer months so checking Orient point or the New London, CT schedules might be worthwhile for the Friday departure.
I too agree with the previous posts. During the week you can get one night stays at some places, even July/August. One time we did luck out and get a very last second over night stay on a weekend in August. Most of the hotels keep in contact with one another and will know if a room somewhere is available. But it is a risk.
JDP