Is the request to the image giving a 404 or a 500 error, or what?
Do you have the CFFileServlet mapping in your web.xml file (it'd be there by default unless you'd been monkeying with it).
You don't really give us much to go on: "it's not working" is not a very good problem description.
Possibly read this: http://forums.adobe.com/thread/607238
I am having the exact same problem. I recently upgraded from CF 9 to CF 10 and my captchas suddenly stopped working, so far on 3 boxes, 1 mac, 2 linux. I imported existing settings from 9 for all 3 and I confirmed that when I change my <cfimage> call so that it writes the image to a destination I choose it works, I can no longer use the method of having it output to the browser, so something is definately not right.
Also, I searched both my old CF9 web.xml file and my CF10 web.xml file, both have the CFFileServlet sections.
I know that this is old, but I ran into a similar problem.
Turned out that there was a URL Rewrite rule in place for SEO that was supposed to set the URL to lower case characters. This broke "/CFFileServlet" the ColdFusion settings. Apparently it is case specific.
Some navigation systems also use !File or !Directory evaluations that will break cfimage. This page has info regarding that problem:
Check your SEO URL Rewrites. It may be causing the problem.
I ran into this under ColdFusion 10,0,13,287689, JVM 1.7.0_25. We use a rewrite in this setup, but it showed to not get in the way. The troubleshooting process was to view the image sourced in the page directly from the server to get an actual error. Then it was noted that the wsconfig number being referanced in the IIS 500 error was not the same as that allowed in the site's ISAPI Filters list (being one number off). The jakarta mapping was pointed to wsconfig 3, and once pointed to the correct wsconfig folder, the image creation/display worked without issue. I did setup CFFileServlet mapping, and it remains in place. I am not certain if it was needed in the resolution.