Has anyone tried this "poor mans" homemade electric bike idea?
No I've not done that, and it would likely be a bad choice for an ebike conversion due to lifespan of motor.

Electric motors have an optimum rotational speed which depends on their design and the number of coils on the armature.

Your wiper motor will have a speed of around 2000 rpm, and it will drive a worm gear - see the excellent photo in @Frisbee's answer.  That worm gear will drop the output RPM to 1/50th  or about 40 RPM.  The output will be a low speed and high torque.  This means the motor wants to spin at a fixed speed, not faster and not slower.

A wiper is also driving a cable or rod system that has half a kilogram of wiper blades attached, compared to the weight of a bike plus rider.  So there's the duty cycle to consider too-how hard the motor has to work.  Remember its a brush based permanent magnet motor intended for forward-backward-stop duty in a relatively cold environment, so running it hard out is not a good idea.

Finally, you'll need to fab up some way to mount the motor and apply drive to the bike.  Your options are


Front or rear hub - not suitable for home job, need to build motor into wheel hub
Tyre drive - possible.  The motor has a roller and pushes that against the tyre creating drive.  Wears out your tyre in triple-quick time.
Mid drive - Attach the motor to the drive chain by a cog and mounting plates.  Downside this messes with your legroom, but may allow the use of rear gears still. 
Offside drive - Some of the liquid motor kits add another drive cog on the left-side of the rear wheel, and run a second chain.  These can't pull away from stopped because they lack a clutch, so you need to pedal up to speed before engaging the liquid engine.    This technique would leave the freewheel clicking when pedals are not providing drive.


You also need to consider batteries for an ebike.  Mine had 3x 12AH 12V batteries mounted in the frame, and all up the bike was about 28 kilograms.
More details at http://www.electric-bike-kit-forum.com/post2249.html 

If you wish to electrify a bike, consider a kitset from somewhere like  http://www.electric-bike-kit.com/hill-topper.aspx   The $800 USD ranger seems decent, and you can revert your bike to normal by taking the kit off.

It looks like you're not the first to have this idea though.... http://www.autospeed.com/cms/article.html?&amp;title=Building-an-Electric-Bike-Part-2&amp;A=111538 
Note he had to build an airscoop to cool the motor, and a powered fan for when stopped.  That's a friction drive too, so uses up tyres fast.

Finally - your locality will have some restriction on the maximum power motor you can fit before your bike becomes classified as a motorbike.  In New Zealand its something like 250-300 watts which is good for about 20 km/h maximum.

Now I can exceed my electric bike speeds by riding my road bike, so I sold the ebike parts and use the old MTB for towing my bike trailer.  Consider upgrading your own motor by biking more for health and fitness.