What version are you trying to run on?
setDividerDrawable() takes the id of the drawable you want to use for your dividers. The argument you are passing is actually used in conjunction with setShowDividers(), which tells it where to draw the dividers.
Replace...
tabhost.getTabWidget().setDividerDrawable(TabWidget.SHOW_DIVIDER_MIDDLE);
with...
tabhost.getTabWidget().setShowDividers(TabWidget.SHOW_DIVIDER_MIDDLE);
tabhost.getTabWidget().setDividerDrawable( ... ); //id of your drawble resource here