After some poking around I found enough evidence that this was not possibly using dojo's current widgets but that it would be easy enough to modify. Last night I hacked together a widget using ComboBox as a base. The code is ugly but it works. I would love to either get this cleaned up as a proper widget or the feature moved into the ComboBox core. Anybody out there willing to help me with this process?

This subject is also referenced at http://trac.dojotoolkit.org/ticket/2589

dojo.require("dijit.form.ComboBox");
dojo.require("dojo.data.ItemFileReadStore");
dojo.require("dijit.form.ValidationTextBox");

dojo.addOnLoad(function() {
	dojo.provide("dijit.quelib.Tagger");

	dojo.declare(
		"dijit.quelib.Tagger",
		[dijit.form.ValidationTextBox, dijit.form.ComboBoxMixin],
		{
			delimiter: ",",
			_previousMatches: false,

			setValue: function (value) {
				if (this.delimiter && value.length != 0) {
					value = value+this.delimiter+" ";
					arguments[0] = this._addPreviousMatches(value);
				}
				this.inherited("setValue", arguments);
			},

			_addPreviousMatches: function (text) {
				if (this._previousMatches) {
					if (!text.match(new RegExp("^"+this._previousMatches))) {
						text = this._previousMatches+text;
					}
					text = this._cleanupDelimiters(text);
				}
				return text;
			},

			_cleanupDelimiters: function (text) {
				if (this.delimiter) {
					text = text.replace(new RegExp("  +"), " ");
					text = text.replace(new RegExp("^ *"+this.delimiter+"* *"), "");
					text = text.replace(new RegExp(this.delimiter+" *"+this.delimiter), this.delimiter);
				}
				return text;
			},
			
			_autoCompleteText: function (text) {
				arguments[0] = this._addPreviousMatches(text);
				this.inherited("_autoCompleteText", arguments);
			},

			_startSearch: function (text) {
				text = this._cleanupDelimiters(text);
				var re = new RegExp("^.*"+this.delimiter+" *");
				if (this._previousMatches = text.match(re)) {
					arguments[0] = text.replace(re, "");
				}
				this.inherited("_startSearch", arguments);
			},
		}
	);
});

As of r11639 this widget as been added to dojox under the name MultiComboBox. See http://trac.dojotoolkit.org/ticket/2589 for details and thanks to dante for getting this done.