I confirm this bug on my Feisty 7.04 with lastests updates.

try :
sudo module-assistant auto-install sl-modem-source

Other methods also fail, i try too :
method 1 : make and make install  (FAIL)
method 2 : fakeroot ./debian/rules kdist KVERS=`uname -r` KSRC=/usr/src/linux-headers-`uname -r`  (FAIL)

All with the same error report :
dh_testdir                                                                 ↑ 
 │ dh_testroot                                                                ▮ 
 │ rm -f build-arch-stamp build-indep-stamp configure-stamp                   ▒ 
 │ # Add here commands to clean up after the build process.                   ▒ 
 │ /usr/bin/make clean SUPPORT_ALSA=1                                         ▒ 
 │ make[1]: Entrando no diretório `/usr/src/modules/sl-modem'                 ▒ 
 │ make[1]: Saindo do diretório `/usr/src/modules/sl-modem'                   ▒ 
 │ cd modem; /usr/bin/make clean SUPPORT_ALSA=1                               ▒ 
 │ make[1]: Entrando no diretório `/usr/src/modules/sl-modem/modem'           ▒ 
 │ make[1]: Saindo do diretório `/usr/src/modules/sl-modem/modem'             ▒ 
 │ dh_clean                                                                   ▒ 
 │ /usr/bin/make -C drivers clean                                             ▒ 
 │ make[1]: Entrando no diretório `/usr/src/modules/sl-modem/drivers'         ▒ 
 │ rm -f kernel-ver slamr.o slusb.o slamr.ko slusb.ko *st7554.o               ▒ 
 │ amrmo_init.o sysdep_amr.o *.mod.* .*.cmd *~       
rm -f -r .tmp_versions                                                     ↑ 
 │ make[1]: Saindo do diretório `/usr/src/modules/sl-modem/drivers'           ▒ 
 │ hostname: Unknown host                                                     ▒ 
 │ /usr/bin/make  -f debian/rules kdist_clean kdist_config binary-modules     ▒ 
 │ hostname: Unknown host                                                     ▮ 
 │ make[1]: Entrando no diretório `/usr/src/modules/sl-modem'                 ▒ 
 │ dh_testdir                                                                 ▒ 
 │ dh_testroot                                                                ▒ 
 │ rm -f build-arch-stamp build-indep-stamp configure-stamp                   ▒ 
 │ # Add here commands to clean up after the build process.                   ▒ 
 │ /usr/bin/make clean SUPPORT_ALSA=1                                         ▒ 
 │ make[2]: Entrando no diretório `/usr/src/modules/sl-modem'                 ▒ 
 │ make[2]: *** Sem regra para processar o alvo `clean'.  Pare.               ▒ 
 │ make[2]: Saindo do diretório `/usr/src/modules/sl-modem'                   ▒ 
 │ make[1]: [clean] Erro 2 (ignorado)     
cd modem; /usr/bin/make clean SUPPORT_ALSA=1                               ↑ 
 │ make[2]: Entrando no diretório `/usr/src/modules/sl-modem/modem'           ▒ 
 │ make[2]: *** Sem regra para processar o alvo `clean'.  Pare.               ▒ 
 │ make[2]: Saindo do diretório `/usr/src/modules/sl-modem/modem'             ▒ 
 │ make[1]: [clean] Erro 2 (ignorado)                                         ▒ 
 │ dh_clean                                                                   ▒ 
 │ /usr/bin/make -C drivers clean                                             ▒ 
 │ make[2]: Entrando no diretório `/usr/src/modules/sl-modem/drivers'         ▮ 
 │ rm -f kernel-ver slamr.o slusb.o slamr.ko slusb.ko *st7554.o               ▒ 
 │ amrmo_init.o sysdep_amr.o *.mod.* .*.cmd *~                                ▒ 
 │ rm -f -r .tmp_versions                                                     ▒ 
 │ make[2]: Saindo do diretório `/usr/src/modules/sl-modem/drivers'           ▒ 
 │ for templ in                                                               ▒ 
 │ /usr/src/modules/sl-modem/debian/sl-modem-modules-_KVERS_.postinst         ▒ 
 │ /usr/src/modules/sl-modem/debian/sl-modem-modules-_KVERS_.postinst.module
 s.in; do \                                                                 ↑ 
 │     cp $templ `echo $templ | sed -e 's/_KVERS_/2.6.20-16-generic/g'` ; \   ▒ 
 │   done                                                                     ▒ 
 │ for templ in `ls debian/*.modules.in` ; do \                               ▒ 
 │     test -e ${templ%.modules.in}.backup || cp ${templ%.modules.in}         ▒ 
 │ ${templ%.modules.in}.backup 2>/dev/null || true; \                         ▒ 
 │     sed -e 's/##KVERS##/2.6.20-16-generic/g                                ▒ 
 │ ;s/#KVERS#/2.6.20-16-generic/g ; s/_KVERS_/2.6.20-16-generic/g ;           ▒ 
 │ s/##KDREV##/2.6.20-16.28/g ; s/#KDREV#/2.6.20-16.28/g ;                    ▒ 
 │ s/_KDREV_/2.6.20-16.28/g  ' < $templ > ${templ%.modules.in}; \             ▒ 
 │   done                                                                     ▮ 
 │ dh_clean -k                                                                ▒ 
 │ dh_installdirs lib/modules/2.6.20-16-generic/misc usr/lib/sl-modem         ▒ 
 │ if ! test -e drivers/Makefile ; then echo "Please update the package,      ▒ 
 │ extract the tarball!"; exit 1 ; fi 
/usr/bin/make -C drivers KERNEL_DIR=/lib/modules/2.6.20-16-generic/build   ↑ 
 │ KVERS=2.6.20-16-generic                                                    ▒ 
 │ make[2]: Entrando no diretório `/usr/src/modules/sl-modem/drivers'         ▒ 
 │ gcc-4.1 -I/lib/modules/2.6.20-16-generic/build/include -o kernel-ver       ▒ 
 │ kernel-ver.c                                                               ▒ 
 │ kernel-ver.c: In function ‘main’:                                          ▒ 
 │ kernel-ver.c:11: error: ‘UTS_RELEASE’ undeclared (first use in this        ▒ 
 │ function)                                                                  ▒ 
 │ kernel-ver.c:11: error: (Each undeclared identifier is reported only once  ▒ 
 │ kernel-ver.c:11: error: for each function it appears in.)                  ▒ 
 │ make[2]: ** [kernel-ver] Erro 1                                            ▒ 
 │ make[2]: Saindo do diretório `/usr/src/modules/sl-modem/drivers'           ▒ 
 │ make[1]: ** [binary-modules] Erro 2                                        ▒ 
 │ make[1]: Saindo do diretório `/usr/src/modules/sl-modem'                   ▮ 
 │ make: ** [kdist_build] Erro 2