A really brief note on the (great) installation instructions that I'm embarassed to say tied me up.
I installed OS, downloaded zaptel and asterisk as directed. I had a minor problem with one of the X100Ps (IRQ conflict, I think) and so restarted without completing the zaptel compilation / 'make install' in order. Installed and config'd asterisk with internal SIP phones to test installation and went back and fixed the zaptel installation second.
Everything seemed to be fine: zttool & /proc/zaptel reported devices were OK, configs were stock (as per examples), asterisk was working fine, but I couldn't get either dial-tone on the FXS channel phones or get a response from asterisk on dial-in on the FXO channel.
I figured out finally that the chan_zap.so hadn't been compiled during the asterisk install (due to the unsatisfied dependency on the initial zaptel compile). This is easily verified in asterisk itself using the zap * commands (ie 'zap show channels') - if they're not there / not available, it's not installed.
A simple re-compile/install (in order!) of zaptel and asterisk patched everything up nicely and didn't even disrupt the config files. Sorry about doubting you, asterisk.