BES supports moving more than one user in a domain to an alternate server as shown in the image below.
Does this mean I could achieve some level of redundancy by moving users from one BES server to another?
Is this a reliable approach with no impact on the end user? (aside from the slight delay in spinning up a new agent, during which time they "Red X")
Would this work in a geographically redundant installation?
You wouldn't achieve redundancy, because they would then be attached to that new server which could also have a problem. You could definitely split your eggs to different baskets this way, and if the server was online you could move them to the other server, but the idea behind most redundant configurations is to maintain availability without the other server. You can setup BES in a highly available way with server pairs. This will also require a highly available database to achieve true redundancy.
As for the second part of that, moving users from one server to another is generally not noticed, but can occasionally require that the user re-configure their device.
http://us.blackberry.com/ataglance/get_the_facts/BES5_High_Availability.pdf
Take a look at the BlackBerry explanation above.
