At the time this question was asked, this was not something RDS supported.
Subsequently, though, AWS announced point-and-click copy of Amazon RDS DB Snapshots across AWS Regions.
To launch an instance from a snapshot in a different region, you have to first copy the snapshot from the region where it was created and stored, into the target region.
In the RDS console, from the origin region, choose "Snapshots," then select the snapshot you want to copy, then click "Copy Snapshot." You will be given a choice of the destination region for the snapshot copy.
After the copy is complete, you'll see the snapshot under "Snapshots" in the target region. From there, you should be able to use that snapshot to create a new instance.
Another option now available is cross-region replication, which allows a live replica to be created in one region, from a master in a different region.
This is relevant, because it could be used for the same purpose of moving a master server to a different region. In this scenario, the master could be migrated from one region to another with minimum downtime by first setting up a cross-region replica in the desired target region, and once the target RDS instance had been created and synchronized to the master, you would disconnect the application from the old master, and then convert the new replica in the new region into a standalone master server, by choosing "Promote Read Replica" from "Instance Actions" in the console, which would sever the connection between the replica and its old master, and allow direct write access to it, since it would now be the new master.