When using an ORM, what are some things to watch out for in your database design
Go through standard normalization techniques, mainly to 3rd normal form. Some ORMs can pick up on the relationships defined at the database level so you won't have to manually.

I would really go the other way and ask what should I know about ORMs. Make sure you know:
 - How do I profile the queries?
 - How can I override the ORM and write my own query?
 - Can I use stored procs instead?

The database should be ORM agnostic since, more than likely, it will outlast the app.