At some point in your TabHost, you may want to call some methods found in your Activity classes. You can actually but not by instantiating the Activity class.
Using the LocalActivityManager class, you can get a reference to the activity object using this code.
String tabTag = getTabHost().getCurrentTabTag();
MyActivity a = (MyActivity) getLocalActivityManager().getActivity(tabTag);
From there, you can call the method. Just make sure it is set to public ;). Oh yeah, be sure you have your tag variables as global so you can get access to those spec tabs anywhere, anytime in case you want to gain access to your Activity classes.