I see there are others that have encountered this problem recently.  I will add so you can see they are apparently not isolated cases.

In my case, users recently began reporting unable to enter email on our website using Safari.  When I tried it (I downloaded Safare to test, since I had not installed before), I also got failure.  In my case the error reads: "Invalid entries found.  Please correct and try again"

My FF & IE work OK with verification, Safari works when I remove verification

I googled "Captcha on Safari" and found several hits like this.  One apparentlly found their fix by upgrading to Captcha version to 5.x-3.0-rc1. That website was http://drupal.org/node/169381.

Hope we can find the answer to this.