To get debugging information for glibc, you need to install the
libc0.3-dbg package. At the place GDB looks for debugging
symbols by default (/usr/lib/debug/lib/), Debian's libc0.3-dbg stores only
frame unwind information. If you want to step into glibc while debugging, you
need to add LD_LIBRARY_PATH=/usr/lib/debug to GDB's environment. If that
still does not work, try LD_PRELOAD=/usr/lib/debug/libc.so.0.3 instead. You
can add to GDB's environment via set env FOO bar from the GDB command line.
It seems that this is no longer needed with the Debian glibc 2.5 packages, but I may as well be wrong. Have to check that again.