Ok. as I said before, the problem is that SME Server kernel are not in sync with CentOS ones. If you want to install FreePBX, you'll need to update your kernel to the latest available from smeupdates-testing (2.6.9-89.0.20.EL):
yum --enablerepo=smeupdates-tesing update kernel kernel-smp
Then, you should be able to install dahdi. Of course, installing packages from smeupdates-testing shouldn't be done directly on production system, so be carefull. I use those kernel (from smeupdates-testing) on several servers without problem but...
Regards