Hi Chas,
I'm not really familiar with CS4/ACR settings but from your screen snap, it does seem that the original image pixel size is being maintained (no re-sizing), ie. 2848x4272 pixels. The 600ppi is, I think, just a tagged value on the image. This, in reality is meaningless, because an IMAGE only has a pixel size, it has no inches so ppi is only relevant to prints.
There is often confusion about this matter so you need to think in Qimage terms for printing, linear dimension for prints.

Your earlier post (reply #21)
talked about so I just want to make sure, the image you use from ACR in Qimage is still 2848x4272 pixels?

Assuming the 600ppi is a tag, IF you used Qimage's "Original size" option, your image would print at 2848/600 x 4272/600 = 4.75" x 7.12". There are a few guys who like to work that way, but it's much easier to use linear (inch) dimensions in Qimage, after all that's what you need to specify for a print size.
In a word, the 600ppi in ACR does not matter, if it's only a tag. (perhaps someone can confirm that)
Accept what Q tells you is the driver's native resolution, (600 for HP & Canon, 720 for Epson), or lower if at a lower than maximum driver quality setting.
I hope we are nearly there now, sorry to go on about this but I had the feeling there was something that was causing confusion - for both of us 
Terry

This is DPI, the printer nozzle spray pattern number, not to be confused with PPI, print pixel resolution.
On my Epson, all the "Photo" quality settings are 720ppi with dpi of 1440, 2880 and 5760.
Terry.

Seth, thank you,
This was the bit of information I did not have in trying to help Chas in a recent post I made. My suspicions are confirmed, he has been re-sizing in CS 
And thank you for emphasising this most important point; only one interpolation is needed for best quality and that's the Qimage one. 

Also for Chas,
That web site is defunct as far as Mike's articles are concerned, they are all here on this one. I did provide the links to similar articles on an earlier post and the one above is there too.
Terry.