Cisco Wireless :: AP541N Clustering And Roaming Clarifications

I'm looking to deploy a 2504 controller and some AP1142s but would like to provide the client with an alternative, lower initial-cost option in my proposal.  I've been researching the AP541N access points but several areas of their implementation seem unclear to me.
 
Clustering: For the clustering feature is it necessary for the network to have other SBCS components (500 series platforms)? All of their other features are either unapplicable to or already implemented in the network in question; if I have to add another appliance I would rather go the LWAPP route and use a true WLC.
 
What are the cababilities of clustering? Can I implement some form of wireless resilience by spacing APs closer together than necessary and they will lower transmit dBm and intelligently respond to attempt to cover a new cell where an AP has gone down?
 
What about roaming? I believe Cisco advertises this as part of a small business voice solution. A client roaming between two access points in the same mobility group (cluster?) on a wireless voip phone should be able to keep connection, as it's analogus to some critical UDP communications that are going to take place on the clients.