It all depends upon how you code it Nethy. If you force
However there are a couple of ways to make it accessible for real people, as well as accessible for the engines.
A more sophisticated, although not that much harder way to do it is have normal links and no onClicks at all. Then have your Ajax script capture the click and subplant the href call if the js/ajax routine fires, ensuring the visitor is on a js enabled browser.
It's actually a lot harder to explain than it is to do.
So how about an example from one of the best and easiest to understand Ajax books I've seen out there: Bulletproof Ajax by Jeremy Keith. As part of the book and part of his site he's cobbled together a quick example of an Ajax web store
application that not only works, but is 100% search engine friendly. Go ahead and try it with JS disabled. The only difference you'll see is the page actually reloading instead of just a section of the page being refreshed by Ajax.
Jeremy even makes the source files available for free
if you want to grab 'em and play with 'em. The hijax.js file is the one that does the heavy lifting of capturing the clicks on href's and form's.