Whilst debugging it in Firebug, I noticed that a new "callback" parameter is passed to the server, which you didn't have with jQuery 1.4.4. After a bit of googling I found a comment on a mailing list that you need to include this callback in your server response.
Previously, the server side script just had to return true or false, so this worked fine:
<cfset result = "true"> <cfoutput>#result#</cfoutput>
With jQuery 1.5, you need to do this:
<cfset result = "true"> <cfoutput>#url.callback#(#result#)</cfoutput>
Hope that saves someone some head scratching!
Update: This is fixed in the jQuery plugin repo on github: