subsubsection shortname seems discarded
it seems that the "short name" of subsubsections is used if and only if the real name is empty (hence will not appear in the TOC). I tested it with the version of beamer provided by texlive2009; although, by looking at the code, it seems that the behaviour would be similar with the current version.
In the file beamerbasesection.sty, in the definition of \beamer@subsubsection, lines 329-330 that add the info into the nav file use the \subsubsecname which is defined as the long title #2 (except when empty), instead of
#1 (the short title).
Mathieu Sassolas (mathieu DOT sassolas AT lip6 DOT fr)
PS: I know that subsubsections are discouraged.