I'm currently working on digitizing a number of art books using a copy stand and
digital camera for the Wikimedia Commons, at the highest possible quality.
(Using a scanner is not an option.) Invariably, the images will come out
slightly rotated, and need to be correctively transformed back, using the
Rotation tool.

My problem is that while using Cubic resampling provides acceptable results for
paintings, it results in blurring and loss of the detail in line drawings. I've
resorted to using PanoTools to do the transformation, but there's no reason why
Gimp shouldn't include it as well.

Various filters are available that will provide much better resolution. See some
 examples of what the higher-quality filters can do compared to the standard
bilinear and bicubic interpolators.

http://photocreations.ca/interpolator/

The filters are defined in the PanoTools distribution, in the file 'resample.c',
which is on SourceForge here:

http://cvs.sourceforge.net/viewcvs.py/panotools/libpano/resample.c?rev=1.8&view=auto

The 'Lanczos' filter used by many applications is just a windows Sinc filter,
which this provides.

While the higher-quality filters require significantly more time to run, the
benefits are very real and noticeable. For the highest possible image quality,
it would be very, very useful to have these resampling filters.

(Not to mention, of course, that this is something that I don't think Photoshop
provides.)