My bad, I had looked at _buffered_raw_seek, not buffered_seek >_<
Indeed, the code is OK in both trunk _io an _pyio modules.
And the SEEK_CUR flag (value : 1) seems more than sufficiently tested in test_io actually, for example in the function write_ops() :
http://svn.python.org/view/python/trunk/Lib/test/test_io.py?view=markup

So concerning python2.6, isn't that just possible to backport _pyio (and its test suite) to it (renamed as io.py) ? They seem to offer the same functionality, except that _pyio is much more robust than io.py (the code around seek(), in particular, is much more complete).
Just tell me, else I'll gather a patch for the seek() problem.