Hey, following the report on xfce mailing list, I open a bug here. Basically, when one runs Terminal, instead of having a new item in the tasklist with “Terminal” (default title) only […] appears. When opening another Terminal window, changing directories, executing commands (thus changing the window title), nothing change. But when opening another soft window (like Thunar), the task list is updated and the correct content appears. I can't really say what triggerred the problem, maybe a GTK+ or a vte (if it's only Terminal related) upgrade. This is with Xfce 4.6.2.
Ok, after few more tests, it's not related to Terminal, but to the first window of anything. It's always […] until another window (a different instance, because if it's the same soft it'll be grouped).
Most likely fixed in the 4.7 panel.