The key requisites were:

• should be able to manage the values for the picklist using the same interface as regular picklists
• should not have to customize the code (just paste the base code and add one line to initialize the picklist)
• does not disrupt tab navigation (I did not 100% succeed on that one because you can only tab forward, not backward)

To make it work I simply created 2 attributes: "HobbiesList" points to the picklist, and "Hobbies" saves the actual values (this wastes a field in the contact table but we only lose 4 bytes).  Both attributes are present on the form but at runtime the HobbiesList is hidden and converted to a multiple selection list to be displayed only when you tab into the Hobbies textbox.

This is the javascript I used in the "Load" event on the form.  This part is generic and does not have to be customized – I just paste it at the beginning of the code.  I struggled a bit to get the positioning right, the rest is pretty classic stuff:

// make the textbox act as a multi select picklist including the elements of the 
// given select control
function makeMultiSelectPicklist(txt, pkl){
    txt.associatedPicklist = pkl;
    pkl.associatedTextbox = txt;
    txt.readonly = true;

    // hide containing row
    for(var p = pkl; p ; p = p.parentNode){
    if(p.tagName == "TR"){
        // hide the entire row (the picklist will be moved out)
        p.style.display = "none";
        break;
    }
    }

    // setup position for the multi select picklist
    var container = document.createElement("span");
    container.style.position = "relative";
    container.style.top = "0px";
    container.style.left = "0px";
    container.style.width = "0px";
    container.style.height = "0px";
    txt.parentNode.insertBefore(container, txt);
    container.insertBefore(pkl, null);
    pkl.style.display = "none";
    pkl.style.position = "absolute";
    pkl.style.top = "0px";
    pkl.style.left = "0px";
    pkl.multiple = "true";

    txt.onfocus = function(e) {
    var txt = this;
    var pkl = txt.associatedPicklist;
    var values = "|" + txt.value.replace(/; /g, "|") + "|";
    pkl.style.display = "block";
    pkl.style.width = txt.offsetWidth + "px";
    for(var i=0; i < pkl.options.length; i++){
        pkl.options[i].selected = new RegExp("\\|" + pkl.options[i].text + "\\|").test(values);
    }
    pkl.focus();
    }
    pkl.onblur = function(e) {
    var pkl = this;
    var txt = pkl.associatedTextbox;

    var s = [];
    for(var i=0; i < pkl.options.length; i++){
        if(pkl.options[i].text && pkl.options[i].selected){
        s.push(pkl.options[i].text);
        }
    }
    txt.value = s.join("; ");
    pkl.style.display = "none";
    }

    // break reference to avoid leak on IE6
    txt = null;
    pkl = null;
}

And finally I add this line at the end of the Load event to initialize the picklist:

makeMultiSelectPicklist(document.all.new_hobbies, document.all.new_hobbieslist);

A little bit more work than in Saleslogix but not too terrible!  And, it works somewhat better for tab navigation.  One thing worth noting – the Javascript editor on the CRM designer is so terrible, it is a lot easier to do the development in a separate file then paste it in.