Viskwal,
I just did a different test with Pages. I opened a Pages layout document with a default 8.5 x 11 template. I dragged a JPEG image onto the template, but did not resize the image -- exported as PDF, and opened with Photoshop CS3 -- no rasterizing was required. Then with the same Pages document, I resized the original JPEG image, and then Exported that as a newly named PDF file. When that file was opened with PS CS3, it was required to be rasterized. Although the second PDF was only modestly larger than the first one, it seems that Pages imparted the "vector info" to explain the resizing, and PS CS3 used that.
Furthermore, when each image (after the PDF was opened) and Saved As a TIFF, both were 300 ppi, but the one where enlargement in Pages took place before Export as PDF becomes a much larger TIFF file than the former.
It would appear to me that Pages is providing an upsizing service that results in interpolation at some point, through some form of vector graphic info, even though using a constant canvas size.
Ernie