this is what Salesforce does - they have publicly visible shards in the url (na11, na14, eu23, etc) and login.salesforce.com redirects you to your shard when it figures out who you are.
In general, over time, load on some shards will increase while others decrease. Migrating a customer from one shard to another will likley cause a short outage for them, and many bugs down the line when they've bookmarked all kinds of things.