In SharePoint 2010, when you create a custom web part, there are times in which it doesn't automatically port over when viewed with a mobile device.
Once this is done, you must 'tell' SharePoint which control to use instead of the original web part, when viewed on a mobile device. Therefore, an entry must be made in the Compat.Browser file, to do this. Also, an entry must be made in the web.config file Safe Controls section to let SharePoint know that this new Mobile Adapter is 'safe'.