I'm not 100% certain, but I believe each DS has to have it's own database.  I don't believe you can tie more than one DS to the same database, at least not with 6.9.  I'm not sure what's changed in 7 though.

I would imagine you could set up two database servers and two DS's and have the SQL database synchronized.  Being as the secondary data center (DR site) is mainly off-line unless the main data center is compromised, this should work.  The point of having a DR is not for load balancing, but to be able to cut over to the exact same named servers in the DR site.

NS is a completely different animal.  NS is more designed to have multiple NS servers that point to the same database instances.  DS 7 is more based on the NS architecture.

I don't do a lot with NS, so I'll leave someone else to really explain that.

 > Can I install multiple Deployment Servers against a single SQL database? 

Yes, you can ;-). DS uses the eXpress database as a default but you can rename the DB to whatever suits you and then modify the System DSN in your Deployment Server to point to the newly named DB.

This can be done multiple times to have many DS databases into a single SQL instance.
> I have 2 datacenters (1 is primary and 1 is Disaster Recovery) and want to run a deployment solution server in each one.  Can I strive to keep both on a single SQL instance, or should I have each DS point to its own SQL instance.

You can have both DS into a single instance or SQL cluster (better if you are considering DR). In term of Database synchronisation a single back up can allow you to restore your jobs and computers history from one (failed) instance to a live one. This shouldn't be too long as the DS databases are never really too large (unless something's wrong).