I looked everywhere I could think to look in PowerPoint to figure out how to do this, and came up with no answers. So, I went on an internet search. The only answer I could find anywhere was an old thread in a PowerPoint group, from Shyam Pillai, a Microsoft MVP (Most Valuable Professional). Shyam says that you cannot change the prompt text in the PowerPoint placeholders, although this was in reference to PowerPoint 97.
If anyone has found that you can do this in later versions of PowerPoint, I'd love to hear back from you. Please leave a comment here. In the meantime, why not check out Shyam Pillai's Office Tips from his MVP Web site.
And ... if anyone at Microsoft is reading this ... you know, this is a really good suggestion for future versions of PowerPoint.