What exactly is the issue? "It doesn't work, has anyone come across this issue" covers a lot of ground. It just sits there not finding the EAS server? It finds it but can't connect? It connects but gives an error? It catches fire and you need to go buy a new phone? Something else? What?
Are there event log messages logged on the Exchange CAS when this phone connects? Are there any in the ISA server logs? Is the other place you've tested this at OK set up the same way as you with regards to ISA server publishing, etc?
Does the mobile phone support the type of certificate you're using? e.g. if you're using a "unified comms" certificate that allows you to protect several host names with subject alternative names, which is typical enough for a Exchange 2007 deployment, will the phone support that type of certificate? More basically than that, does the phone trust the certificate provider?
If you go into the account you're setting this up with in OWA, does the phone details page show anything? (OWA options / mobile devices)
What EAS policies are applied to the user? Are they compatible with what the phone itself can support? e.g. do you require info to be stored encrypted and the phone can't do that so won't sync due to that?