Does a white PowerPoint background strain the eyes?

Yes, Absolutely.

The biggest problem with eye strain when it comes to presentations is
contrast. This is especially the case when you turn down the lights.

Read this: http://www.sangrea.net/ohs_dbase/colour-color.htm

Not only does it strain the eyes... you would lose your audience very quickly...

You will want to use a green to light green to a dark green
background. Green is the easiest color on the eyes. I would use green
to forest green for your background.

Any other color will hurt the eyes. Black and white are the worst.

As a web designer, I know that colors are everything. A pure white
background is very straineous to eye, especially if it is contrasted
against large black objects.

There are a number of easy-going backgrouns included with Powerpoint.
Try and choose a "faded" color. Your best bet for something sleek and
professional would be to go with a Light brown-ish hue. This would
allow both black, white and other various colors to be applied to your
presentation without straining the eyes of your "audience".

For more references or inspiration, you can visit these color charts:
http://www.visibone.com/colorlab
http://www.hypersolutions.org/rgb.html

You can also have a read at this forum post for more information and
color complimentation:
http://www.graphic-forums.net/showthread.php?t=5756

Good luck with your presentation!

Here is a link to many choices of pre-made presentations..

http://office.microsoft.com/en-us/templates/CT011377351033.aspx

Enjoy

I would agree with all the other comments about backgrounds and contrast, etc.
Have you ever been to http://www.istockphoto.com ?  I use this great
little site for just about everything, print media, web design, etc.

If you just do a search for back grounds you should be able to find
some cool stuff.  It's cheap as chippies as well.