HIV can only be transmitted if certain bodily fluids are permitted to enter the bloodstream. These fluids are blood, semen, and possibly saliva. Consequently, in the two scenarios you presented, no risk is apparent, unless you were to somehow come in contact with open sores on an HIV carrier while checking ID or touching arms. This seems unlikely. If you are in a physical altercation with an individual, however, caution is warranted. I am unqualified to comment on preventative measures to be taken in such cases, as I am not in the security business.
I hope this helps,
[This message has been edited by Mort (edited 01-04-99).]