Так, что ли?
<html>
<body>
<textarea id = "postarea" rows = "5" cols = "120" name = "txt"></textarea>
<br><br>
<input type="checkbox" onchange="tag_add(postarea, 'one', this.checked)"/>one<br>
<input type="checkbox" onchange="tag_add(postarea, 'two', this.checked)"/>two<br>
<input type="checkbox" onchange="tag_add(postarea, 'three', this.checked)"/>three<br>
<script>
function tag_add(obj, str, add) {
if( add ) {
if( obj.value ) {
if( obj.value.indexOf( str) < 0 )
obj.value += ";" + str;
} else {
obj.value = str;
}
} else {
var re = new RegExp( "(;)?"+str+"(;)?" );
obj.value = obj.value.replace( re, function( x, l, r ){ return l && r ? ";" : ""; } );
}
}
</script>
</body>
</html>