That would be pretty lousy. Fortunately I haven't seen any evidence to support that idea. kalmn has had her Prius for about four years and hasn't needed new batteries. As of November of 2005 Toyota hadn't sold a replacement battery yet. (The warranty on a Prius covers the battery for 8 years/100,000 miles (or 10 years/150,000 miles in California)).
This enthusiast site vehemently disagrees with the idea that batteries will have to be replaced frequently:
http://john1701a.com/prius/prius-misconceptions.htm#Replace