Create a link on a member profile page next to their picture which says CONTACT <username>. When clicking on this link, use lightbox-style javascript to call up small form with fields:
Name:
Contact Info:
Message:
Upon submit of form, close lightbox window and create a INBOX message for member profile with information. Message will contain all three fields in body and FROM part of message will say CONTACT FORM so the user knows it came from their page.
Please make sure that when placing information into database (user's inbox) you also call function which sends out real email to member email so they get notification that they received message in inbox.