Like this one I coded? :
http://bainbridgemarine.co.uk/Map.aspx
Ps: I didn't do the design, just the javascript / xml handling
Oh and it is doable, but I am not confident in my knowledge of the maps API

so I won't quote for it
More detail: You set a radius around your points latitude and longitude and then request the geocoder tool to check if it falls within that radius.