Domain integration

I have a question for many of you out there. I have to find out a way to join two separate domains (different Forests) for my company. This needs to allow for security rights to work with many web enabled applications, SAP, sharepoint, others. It also needs to be able to support File Shares.

I understand that one method of doing this is to create a trust between the domains. I am trying to avoid that because of all the issues involved with it.

I know that Microsoft has ADFS which allows for a sort of Federated service and then there are Virtual Directories that will handle the LDAP aspects.

What would be a solution to handle share drives and windows shares in particular? I guess id be looking for a way for a user to seamlessly use one user ID (as far as they are concerned) and have that access anything instead of using multiple IDS.

Thanks