For Serendipity, only the frontend (what the visitors see) could be subject to Smarty-Templating. One reason for not utilizing these features in the backend was to maintain stability, ease of change for core developers and reduce migration woes so that the Admin Backend would always be accessible.
What we have now added to the Serendipity 1.2 snapshots (that will soon become public beta and a final release in late Summer) is functionality that allows you to template the backend layout as well as the 'New/Edit Entry' screen. Other functions like category manager, plugin manager etc. will remain hardcoded and eventually changed, because most of their look can already be controlled with CSS only.
To maintain stability and prevent migration problems where Smarty might not be initialized, Serendipity can fall back to the usual PHP-only backend. This is done using a tricky session variable scheme - when Smarty cannot be loaded, a session variable is set, and on the next page call, this variable will force the Serendipity framework to use the fallback routines. Nifty stuff. :-)
Please try out the new theming possibilites and give feedback. The default admin stylesheet can be found in the templates/default/admin/index.tpl and templates/default/admin/entries.tpl templates, and can be copied to your own theme directory as usual.