Like all things graphical and technical, there are always corner cases.
In terms of upsampling, every RIP and every renderer out there upsamples images when necessary, i.e., when the effective resolution of the raster image (native resolution tempered by the magnfication for print or display) is less than the resolution of device on which they are being printed or displayed. And the converse is true as well in terms of downsampling.
In fact, with the exception of one-bit-per pixel monochrome images at exact device resolution, all imagery is resampled during the process of halftoning. You are not avoiding a resampling process by trying to hit some magic number in terms of resolution!!
What you do want to avoid like the plague is the result of cascading resampling of imagery. All raster image upsampling operations are at best speculative in terms of how they “invent” the extra pixels for interpolation. What is used in Adobe Photoshop as well as by other Adobe applications including Acrobat and the Adobe PDF Print Engine RIP technology by default, bicubic interpolation, is generally very effective and high quality. All raster image downsampling is lossy. And if you upsample (interpolate) an image and then downsample it, which pixels do you lose, the original information or the invented information? The results can be quite nasty.
The best example of this problem is that of screen shots. What is the best way to reproduce a screen shot? You will get all type of opinions on this, but what you definitely never should do is to upsample it. (There are some interesting techniques that can be applied to clean up black text in such screen shots, but they don't involve attempts to change resolution.) Simply place the screen shot “as is” and let the screen display software in your application or in Acrobat or the RIP for printing resample the image once during the screening process.
Note that there are various Photoshop plug-in packages that claim miraculous results for upsampling requirements, but these miraculous results are typically representative of situations in which really extreme upsampling is required and not the garden variety that most of us would normally deal with. Of course YMMV.
You also need to be exceptionally careful about any resampling, downsampling or upsampling given the havoc it can cause with images that have had any significant sharpening already applied to them.
Hopefully this cut the confusion a bit ... These issues are probably one of primary reasons one should stay with text rendered via fonts and vector graphics as much as possible before resorting to use of raster images. (BTW, one of these days the geniuses at Microsoft and Apple will learn how to provide a screen shot facility that uses text and vectors in addition to rasters based upon the original display lists provided to the underlying windowing system!)
Well, I guess the upshot is I'd be careful saying there's no quality increase to upsampling, since, at least to me, line art does not feel like a corner case.
(Maybe that was key here was the thresholding operation, i.e. the elimination of the poor interaction between antialiasing and halftoning.)
If I can't get this artwork at 300ppi and am stuck with getting it at 72ppi, there's pretty much no way I am going to get it in vector form! Much as we'd love it.