Yes, it is possible. But, within PowerPoint, you will have to set an action on the Excel object when it is clicked. It's not pretty, but it does work. I am doing this is 2002 (Office XP)
What I did was create 3 columns in Excel and create a formula which was A*B=C. I hid column B, and then copied columns A & C over to PowerPoint.
Then within PowerPoint, I right-clicked on the Excel object I copied over and chose Action Settings. In the Action settings option, you have Edit and Open. I chose Edit, although in this instance they appear to do the same thing. Choose OK and then go to your PPT slide show. When the slide comes up, click on the Excel object. This will take you into Excel with your datasheet showing. Make the change you want (I changed the number in column A). Now close Excel. You will be back to your PPT presentation with the new data reflected. I haven't found a way to do it without bringing up Excel, but at least this keeps you in your rpesentation. The caveat is that you'd need to be sure Excel was installed on that machine.
Hope that helps-