David Pogue did some A-B testing of 13MP images resized to 5MP and 8MP, and nobody can tell the difference:
Pogue’s Posts - Technology - New York Times Blog
This is not a big surprise to me, because we've done tests like these over the past 5 years with Picasa's print engine.
Picasa does Lanczos-3 interpolation to 1/2 the native printer resultion by default when it prints, and a user can request that we use Lanczos-8 at full-res! Needless to say, I think our output looks rather good, and we do better with small-sized images than almost any other app out there.
Many major online print providers still do awful interpolation, and so part of this equation is: what kind of interpolation does the printer driver (or RIP) use when it's printing? I've seen many well-known services do nearest-neighbor resampling, and almost all consumer printers do this. You might not be able to tell, but you probably could if you looked closely.
When we chose a default "low-res" size for Picasaweb, we actually picked 1600x1200 based on some double-blind prints at 8x10.
I'm not kidding: people could not tell the difference between a 2Mpix image printed with great interpolation at 8x10, compared with an 8Mpix image printed at full resolution.
Sounds crazy, but there it is...