Hi Ryan,
Are you writing a new module?
if so, you should be inheriting the "PortalModuleBase" class.
This class will set all of the values you need so you really don't have to call anything.
using "this.TabId" should get you the current tab id. If that doesn't work for some reason, the object "PortalSettings" should already be set for you and you can use "PortalSettings.ActiveTab.TabId" OR if for some reason that is not set (should be), you can get the Portal Settings with the Controller:
PortalSettings ps = PortalsController.GetCurrentPortalSettings();
Then using ps.ActiveTab.TabId should do it.
The first option should work, but if not there are a could more for ya.
Hope that helps!