Accessibility compliance is becoming more in vogue each day. There's not only the need to comply within the US and to abide by the rules set at the 508 Rehabilitation Act, but also a more global approach derived from the widespread distribution of the Internet and the standards set by W3 Schools and its WCAG guidelines.
In my experience, and partly due to the ever changing landscape, clients tend to believe they are providing you with an already compliant native copy to be used as the input for your translation project. More often than not, this ends up being a fiasco if you totally rely on them and do not invest (waste?) time on checking the level of accessibility provided within the source document.
To be honest, I'd rather tell them I prefer to assume we need to prepare it from scratch with a risk of DTP costs skyrocketing (at least from what they might perceive). If not, be ready to make an assessment (you may create your own checklist). At any rate, try checking the most obvious errors (like missing alt text, incorrect tagging or reading order, lack of language setting) to be able to quickly go back to them and convince them it might be better to build it from scratch.
In any case, this is my point of view and I more than welcome other voices in need to speak out.