Some mobile devices (and I say some, because I don't know why this is the case... yet), like the cert better if you export it as .DER encoded. You can go to your servers "certificate" store through mmc.exe and export the public cert as .DER encoded. It's possible this may install on your WM5 device. This one is actually supported, but may still be blocked by the security model on your phone.
A friend of a friend of a friend of a friend used Bernt Lervik's method. I can't say it's a supported method, and as always, messin' around in the registry may wreck your device, but it just might work. I have not tried this method, and it may void support on your device from Microsoft or your provider!
And in other news. Windows Mobile 6 has released, you'll probably start seeing these devices around Christmas time. Looking to update your existing Windows Mobile 5 device? Keep your eyes on your provider and manufacturers websites.... and good luck!