Customizations do not show up in Mobile. You have to write all your customizations again. It's not fun, for the Blackberry you need to have a Java developer on-hand, and for the Windows Mobile, someone with .NET development experience, so I would recommend only supporting one type of device, preferrably the one you have the most in house development experience with, but most companies are running blackberries so you're probably going to want to go that direction.
You can set up real time push sync, so it pushes the changes both way or manual sync. If you have it manual, the user on the device has to initiate a sync for their changes to get sent to the server.