I think there is enough to go on in some of the other threads- you may
want to do a search. I recall a very long thread that had a couple
messages that laid everything out for you. Basically you want to add
some new UI object as a subview to serve as a marker, and as you trap
events such as the map moving, you have to move the marker. And of
course you'll track off-screen markers to see if/when it is time to
make them visible again.
None of the code will be complex I don't think- you just have to be
careful and attentive to details to have everything look seamless.