OK Here is how you get your certificate if you already have exchange working on another device.

1. Get the name of the mail server from your current mobile device. (On Symbian its in the Mail for Exchange "Server" setting).
2. Lets say that this server is "mobileserver.mycompany.co.uk"
3. Use this url in internet explorer address bar like so (adding certserv) https://mobileserver.mycompany.co.uk/certserv

4. You should arrive at the home page of your mail server, and it will say something like "you are about to be redirected to a secure connection to check your identity and credentials"
5. You will notice a padlock at the end of the address bar on IE7.
6. Click the padlock
7. Click "view certificates"
8. Click the "Details" tab
9. You will now see your certifcate details.
10. Click "Copy to File". A Wizard will start up
11. Click "Next"
12. Select the format of your certificate as Base-64-encoded X.509(.CER)
13 Type in the file name you want to save it as leaving off the extension.
14. Click "Next"
15. A nice shiny Certifcate will now be on your computer ready to save to your N900 in what ever means you choose.
16. Use the N900 certificate tool to import the certificate

I dont have N900 so someone else will have to show the steps to import.