Stego via TCP/IP (was Re: crypto wish list)

What, a discussion of cryptography on the cypherpunks list?

aba@dcs.ex.ac.uk (Adam Back) writes:
>>Where is highly sophisticated stego?
>What are our options?
>- Stego in english text.
>- Stego in audio and graphic file formats
>- Stego in Internet Phone protocols.
>- Stego in Internet video conference formats

What about stego in IP itself? It's been awhile since I've looked, but
aren't there some bits one could subvert in the TCP/IP headers themselves?