I don't know exactly what id#, but once you get so high up in the gump id's (like the 52K range [again, not sure, might start lower]) the animations will stop displaying when you sit.
If you have a lot of wearables to add, and you have any jewelry or such items that don't need an animation, add them in the upper range to save the lower range for your regular wearables.
Hope that makes sense.. and hopefully someone else knows the id# the sitting "bug" starts at?
(not really a "bug" of course, just the way the client is coded)
- Jandar
P.S. Anyone have a compiled list of all the 'free' gump id #'s that can be used for wearables?