Set of standard accessibility guidelines for online maps?

The W3C standard that is most relevant is WCAG 2.0 AA. Essentially, it's a matter of evaluating if something is Perceivable, Operable, Understandable & Robust.

I really don't know how you'd be able to navigate a map without a keyboard (and there are a lot of users who don't use mice). You could possibly make the navigation appear when the focus is on the map. However, you'd also want to make sure that the map is operational.

I haven't heard of a common set of standards for map accessibility. I think right now the expectations are pretty low.

Screen reader users are one important case, but I think in the mapping world many people start there, decide it can't be done and move on. There are people with low vision, colour blindness, mobility chalenges, etc.

There is an interesting list in OpenStreet Map's Wiki.

Something to be mindful of is who the targeted audience is. Some users may not understand the map to pan control, whereas others may. The Flex API widgets/fonts are small and are not very easy to enlarge with a web browser. Here is an interesting article.

Trends in Web Mapping: It’s all about usability

The W3C Research and Development Working Group discusses detailed guidlines in https://www.w3.org/WAI/RD/wiki/Accessible_Maps