When I come across a web site that isn’t accessible, I usually just leave. However, if I really want to access that site, I can often do so through the mobile interface, assuming it is available.
This work around isn’t perfect, as the mobile site almost never has all the content or features of the main site. However, limited access is better than no access at all.
The other, more serious problem I encounter when using this hack is caused by some sites that recognize that I’m using a browser on a PC. It is possible to fool the server, but it’s not easy.
The fact that this work around exists is of course no excuse for not making your main site accessible. However, if your main site is inaccessible, and you have a mobile version of your site, at the very least, don’t block it from being able to be viewed in Firefox or IE.
Remember also that a mobile site is no substitute for making your main site accessible, unless it simply isn’t feasible, you provide as much content and functionality as possible on the mobile site, and you place a prominant link directing users with accessibility needs to the alternate site.
Finally, just because a site works on a mobile device doesn’t mean that it is accessible, though the odds are much greater that it will be. Nevertheless, the standard web accessibility advice and guidelines still apply.