The easiest way is to use the native Windows Ctrl-PrtScn button combination. This creates a bitmap image of the current Window in the clipboard.
You then switch to PowerPoint and paste the image in.
Of course, this leaves you with the MS Project interface in the image as well. So you then need to trim the image in Powerpoint. Alternatively, you can go to full-screen in Project before doing Ctrl-PrtScn.
If that is too many steps for you. I would recommend using a screen capture tool. If you are a Microsoft Office user, see if you have access to Microsoft OneNote which includes a quick screen-grab shortcut. Windows 7 also has the Snipping Tool which might be helpful.
There is also (or at least there used to be - I can't check whether it is still there in 2010 at the moment) a snapshot button in Project itself which captures an image of the current chart without Project's interface. The button looks like a camera.
Another possible alternative is to print to PDF. This would then let you either insert direct to PowerPoint or open and copy a selected part of the image. Though this isn't at all easier than doing a screen capture!
Finally, you can, of course, simply include an MS Project plan as an object in PowerPoint though this tends to make the PowerPoint files rather large and unwieldy.
Thanks go to @Shivaranjan for the information that, in Project 2010 the camera button is now replaced as 'Copy Picture' as shown in the screenshots in the following links.
First click the copy picture button
and then choose how to export it