Is there an easy way to replicate MySQL data? My software is deployed in various cities, each have a database, but they have different authorizations such as:

              central DB
                 /\
                /  \
              DB1  DB2
              /\    /\
             a  b  c  d


Databases a &amp; b need replication to DB1.  Databases DB1 &amp; DB2 need  replication to central DB.

How can I accomplish this?
I have two suggestions
Commercial Product
There is a product called ScaleBase that can handle Geographic replication topologies.
Handcrafted Solution
If you want to set up Geographic Replication using MySQL Replication, you will need to make the MySQL Instance in each Data Center resilient. Here is what thinking:
IDEA #1 : Local Data Redundancy
In each city, setup DRBD with MySQL running on the DRBD Primary
IDEA #2 : Use Distribution Masters
Having a MySQL Replication Master handle the traffic of SQL statement between Data Center could possibly add some intermittent behavior in terms of Replication Lag. If you run another MySQL Instance within the same Data Center whose sole purpose is to ship binary logs to another Data Center, that can be setup in what some call a BLACKHOLE slave. That MySQL Instance contains no actual MySQL. It simply has binary logs and relay logs sued for shipping to multiple slaves. I have discussed using this type of Slave in StackOverflow.
CAVEAT
The two ideas can be combined. The only drawback for using IDEA #2 is doing ALTER TABLE. If any major changes are to be done to table layouts, make sure the Distribution Masters do not change from the BLACKHOLE Storage Engine because this could cascade to the other Data Center and possibly destroy data.
EPILOGUE
I wrote about setting up such topologies before

Aug 20, 2012 : Best solution for cross-datacenter MySQL master-slave replication
Aug 20, 2012 : How to use MySQL Server 5.6 for GeoIP replication?
Jun 25, 2012 : Mysql database replication on different vlan/subnet/another site
Dec 19, 2011 : Best way to setup master to multi master replication
Dec 17, 2011 : Mysql Master-Master Replication Topologies on &gt;2 machines
Aug 18, 2011 : With MySQL Replication, what level of resilience is possible?
Jun 01, 2011 : What can we do in MySQL 5.0 Replication to address bandwidth concerns?
Mar 29, 2011 : MySQL high availability, failover and replication with Latency

Give it a Try !!!