Pidgin is a bit picky about its icons and, if the theme doesn't provide icons under the non-standard names it uses, it falls back to the application icon in all cases.
This means that, while using the Elementary outside something like Unity (for example, as the inherited base for the default icon theme on Lubuntu), users don't see any indication of held messages or their current account status.
The simplest fix is to symlink the panel/*/user-*-panel.svg icons to status/*/pidgin-tray-*.svg, matching the names for the default set in /usr/share/pixmaps/pidgin/tray/hicolor/. Elementary doesn't have an icon for when there's a message waiting, but falling through to the default application icon will do perfectly well for that case.
In case anyone stumbles across this looking for a quick fix, I wrote a little script to fix it on my system. I'm attaching it, but here's the copy on GitHub in case I find and fix any bugs:
https://gist.github.com/1815976
Instead of adding app-specific icon names to elementary, I think it would be more appropriate to file a bug report against pidgin asking it to use app-agnostic icon names.