Should I keep pg_xlog on the same disk as data if using disk snapshotting? We're running on EBS volumes on EC2.  We're interested in leveraging
EBS snapshotting for backups.  However, does this mean we'd need to
ensure our pg_xlog is on the same EBS volume as our data?

(I believe) the usual reasoning for separating pg_xlog onto a separate
volume is for performance.  However, if they are on different volumes,
the snapshots may be out of sync.
Having the transaction log (pg_xlog) on separate storage can increase write performance on most RDBMS platforms. If yours is a (concurrent) write heavy application, you may see a performance benefit.

If the snapshots are not synchronous however, you could lose data from uncomitted transactions in the event of an unclean restart. I suppose in certain situations a pg_resetxlog could be required after a failure.

You could look into WAL archiving, or even techniques similar to ZFS snapshotting if EBS snapshots have the same effective purpose.