He got the following error:
Secure Connection Failed
The certificate is not trusted because the issuer certificate is unknown.
(Error code: sec_error_unknown_issuer)
I checked in IE7 and FF3, but it worked for me fine.
I also got a screenshot and I assumed the user has FF2.
So I fired on old XP, SP2 virtual machine, installed FF2 and could reproduce the error.
Then I installed FF3 on the VM, and it also had problems.
So I figured, if I updated the machine it will install the root certificate update and the problem would be solved, but I just did not find this a good solution, so I investigated further.
I found out that I missed the issuers intermediate certificate installation on my server. So I did it, and the error disappeared on the unpatched machine as well.
I just had to insert the following in one of the virtual servers configuration: