I did open up both the Redbook-ripped 44.1/16 wav file and the iZotope 64-bit SRC upsampled files in SoundForge 10, and zoomed all the way in, to where the display starts showing points connected by straight lines, and looked at various corresponding points.
Yep, there were visible differences: zero crossings happening at slightly different points, slightly changed peak points, slightly different curves drawn.--TINY TINY differences.
The upsampling algorithm has to put an additional point **somewhere**. As it does so, the algorithm and parameter settings used determine where those extra points go--it doesn't just interpolate on straight lines. And then there are additional corresponding differences again, when the D/A to analog Line Out converts to points to a continuous electrical signal. Under a High_Resolution setting, the Voxengo SPAN frequency analysis tool shows visible differences in the spectra--very small, but SPAN at least knows they are there. Don't have the graphic here, but I'll post it later.