This is due to the way the show/hide tabbar plugin tries to find the widget containing the tabs, which doesn't work anymore when gedit-latex-plugin adds a toolbar to the window.

A solution is to recompile gedit-plugins (the package containing the Show/Hide Tabbar plugin) after a modification: replace line 124 in showtabbar/gedit-showtabbar-plugin.c

container = GTK_CONTAINER (g_list_nth_data (list, 2)); /* HPaned */

by something like:

container = NULL;
gint n, i;
n = g_list_length(list);
for (i=0; i<n; i++)
if (GTK_IS_HPANED (g_list_nth_data (list, i)))
container = GTK_CONTAINER (g_list_nth_data (list, i)); /* HPaned */
if (container == NULL)
return NULL;

I can send a modified plugin library (libshowtabbar.so) compiled for Ubuntu 9.10 by email if someone needs it.