International Consulting Batwrangler
Yesterday, my cousin in Montreal had a bat in his house, so he called me. (He also called some pest control people, but I had a better response time and was on the phone with him when he relocated the bat, which he successfully captured in a plastic container and took outside.)

As a service to everyone else not in my immediate vicinity:

What to do if you discover a bat in your house when you don't have a bat net

1. Don't Panic.
2a. Try opening the windows to let it fly out on its own.
2b. Use your towel: Throw it over the bat. This will either entangle the bat, allowing you to take it outside in the towel, or cover the bat, allowing you to slip a piece of cardboard under the bat so you can then take the towel-bat-cardboard sandwich outside. (Discard the cardboard and wash or discard your towel afterward.)
2c. Alternatively you can place a box, bucket, or can over the bat, and again using cardboard to contain the bat, take it outside. (Discard the cardboard and wash or discard your container afterward.)
2d. Lastly, you can put on a pair of heavy gloves, pick the bat up, and take it outside. Don't do this unless the other methods fail. Bats are delicate and can be broken easily.
3. Take pictures: They make cool "trophies."
4. Try to determine how it entered, and block off the entry. If bats are living in your attic, you must block their entry to the attic with a one-way door to allow them to escape outside. (For more details, try: http://www.batcon.org/binb/doityourself.html)

NEVER handle a bat with your bare hands or with light gloves: you don't want to get bitten, and bats have such small sharp teeth that you might not realize they have broken your skin. If you do get bitten, do not let the bat go. It will have to be killed and tested for rabies.

If you wake in a room with a bat, or if you find a bat in a room with small child, or other person who is unable to tell you if they have been bitten, the bat will also have to be caught and tested.

Very few bats have rabies, but most people who die of rabies, die from bat rabies. Don't take chances. (See: http://www.cdc.gov/ncidod/dvrd/rabies/bats_&_rabies/bats&.htm)

(Just in case you didn't know that information you find on the internet is not necessarily reliable: I am not a professional; Follow these tips at your own risk.)

See also Bat Conservation International's bat handling pages: http://www.batworld.org/found_a_bat/found_a_bat.html

To find a bat rehabilitator, look up your location here: http://www.batworld.org/local_rescue/local_rescue.html

For comprehensive bat management information, take a look at: http://www.npwrc.usgs.gov/resource/1998/housebat/housebat.htm