If you shapeshift while wearing any disguise, you will notice that your form buff will remove the disguise buff. This is because of problems caused by the two buffs when they were allowed to stack. Blizzard didn't like the bugs that were made through these effects stacking even though they were used to have fun most of the time. Items like the Noggenfogger Elixir were then changed to not stack with Druid forms so that it would not cause any future problems.
As of Patch 5.0.4, some vanity shapeshift items will work while mounting (such as moonkin form) but others will be removed upon shapeshift.