Drag radials

As an example, see the link at the end to the "Create a timeline agenda in PresentationZen style" which was done by Garr Reynolds. If you look at the embedded presentation's slides 7-12, you'll see a slight gradient.
A radial gradient background with a lighter center creates a highlight effect. It looks like a glow. You can make your glow bright (white with a much darker color on the outer rim) or subtle (a lighter shade in the center and a darker shade of the same color on the outer rim). Often, the highlight is not in the center of the slide, where it can be distracting and hinder legibility, but in the upper-left or upper-right quadrant of the slide. However, Garr's gradient is in the center.
PowerPoint 2003 doesn't have a true radial gradient but you can get a similar effect by using a From Center setting in a circle. The gradients are less flexibile in 2003, too. You have to use a trick in 2003 to get a decent result. Here are separate instructions for 2003 and 2007.
* PowerPoint 2003
Follow these steps to create a background in PowerPoint 2003:
1. Open the Slide Master (View> Master> Slide Master).
2. Click the Oval tool on the Drawing toolbar at the bottom of your screen, hold down the Shift key to ensure a perfect circle, and drag a large circle across the slide.
3. You'll probably need to reduce the zoom to be able to cover the entire slide, so click the Zoom drop-down list and choose 50%.
4. Use the corner handle of the circle to adjust its size until the entire slide is covered. To move the center to the upper-left corner, say, move the circle up and to the left. Again, enlarge the circle to make sure the entire slide is covered. (Unfortunately, this enlarges the lighter area at the center of the circle, so that center isn't confined to the upper-left corner.)
5. Double-click the circle to open the Format AutoShape dialog box.
6. On the Colors and Lines tab,click the Line Color drop-down list and choose No Line.
7. Click the Fill Color drop-down list and choose Fill Effects.  
8. In the Fill Effects dialog box, click the Gradient tab.
9. Choose the One Color option. You could also create a gradient with the Two Color option.
10. Click the Color 1 drop-down list and choose the color you want, or choose More Colors and specify the color.
11. In the Dark/Light slider, drag the box slightly to the Light side. This will create a gradient of white with your chosen color.
12. In the Shading Styles section, choose From Center.
13. In the Variants section, choose the variant that shows white in the center.
14. Click OK twice to return to your slide. Go into Slide Show view to see if you like the result. You may want to adjust that Dark/Light slider a little to get just the look you want.
15. Return to the Slide Master. In order to see where the slide is, you'll need to make the circle semi-transparent. Double-click the circle and drag the Transparency slider on the Colors and Lines tab to about 50%. When the presentation is done, change the transparency back to 0%.
16. Return to Normal view (View>Normal).
* PowerPoint 2007
In PowerPoint 2007, you can use a similar method of creating a large circle on the slide master, but you don't have to. Instead, you can format the background with a radial gradient background or use the Backgrounds feature, which is essentially a quick way to create a radial background (but with fewer options). First, I describe how to format the background with a radial gradient.
Follow these steps:
1. Open the Slide Master (View tab> Presentation Views group> Slide Master).
2. Click the larger thumbnail of the layouts in the left pane.
3. In the Slide pane, right-click an empty area (not on a placeholder), and choose Format Background.
4. In the Format Background dialog box (Fill category), click the Gradient Fill option.
5. From the Type drop-down list, choose Radial. From the Direction drop-down list, choose one of the two rightmost options (upper-right or upper-left corner).
6. Under the Gradient Stops label, click the Stops drop-down list. If there are 3 stops, choose Stop 3, and click the Remove button.
7. Choose Stop 1. From the Color drop-down list, choose a light shade of your desired color.Read more Drag radials. The Stop Position should be 0%.
8. Choose Stop 2. From the Color drop-down list, choose your desired color. I chose 45% for the Stop Position, but you can drag the slider until you get the result you want.
9. Click Close.
10. Return to Normal view (View tab> Presentation Views group> Normal).
To use the Backgrounds feature, follow these steps:
1. Open the Slide Master (View tab> Presentation Views group> Slide Master).
2. Click the larger thumbnail of the layouts in the left pane.
3. Go to Slide Master tab> Background group> Background Styles and choose one of the options from the bottom two rows.
Note: The color options are based on your theme colors, so you may have to change those to see the color you want. Change one of the two Background color choices.For more info visit Drag radials.