I found a nice read \ explanation how WSP files are deployed into you farm. It tells also that new front end server are automatic updated.
you can read it here: http://blogs.msdn.com/cjohnson/archive/2006/09/11/749105.aspx
I also found this slide in a powerpoint presentation:
http://blogs.macaw.nl/blogs/information_worker_solutions_intranet_extranet_office_system_enterprise_content_management/attachment/1719.ashx#271,31,%20Solution%20Deployment
Pretty cool heh