1) If the page is clipped (due to wide, fixed-width designs in smaller screens, the right side is what gets clipped; if that’s the right navigation then it’s less of a problem on the user who’s probably reading the main text anyway, and thus doesn’t have to joggle the horizontal nav bar.
2) Right side navigation is semantically better for keyboard navigators, especially if no other keyboard aides are used (tabindex, accesskeys, skip links, etc.). Proper use of markup and semantic organization is a Priority 1 accessibility issue.
3) Related to #2, search engines generally give greater weight to the keywords in the top region of a page, so if the semantic order puts a lot of skewed navigation links first (links targeting other pages than what is active) then current page referencing is impacted.

And good point about the right hand convenience issue. I’m actually left handed but picked up right hand mouse use as easy as I did right handed scissors in grade school. Kind of a forced ergonomic thing, I guess, but it just seemed like the more natural way…keeping the cursor on deck on the right side where the scrollbar is anyway.

The “it’s what everybody else does, so people are familiar with it” argument doesn’t really fly with me. People aren’t stupid. If the navigation elements LOOK consistent with what’s on other sites, then people will find it whether its on the left or right (or top — bottom is another story).

The goal shouldn’t be conformity — the goal should be …. well, goal achievement — helping users achieve their goals (why they came to the site) AND achieving the credit union’s goal (getting members to read content, take action, etc.)