Is it not possible for a Yahoo! or Google to show me online (on IM) whenever I am using any of their services? Well.. Must not be a problem. This is when I am actually not logged into IM using either the IM client (or mail as in GMail).
Now, first question : How will I receive a message sent to me when am not logged into IM? This is where toolbar can come into the picture. The toolbar can show a flashing icon when someone is attempting to get in touch with me. I can then (obviously) choose to either login to IM and respond or switch my status to offline. The default will be to show a person offline.
Second question : Usage model / Users. I have no idea who will find this useful or what usage models can be developed around this. If any readers of this post have ideas, leave a comment or blog about ur ideas. Don’t forget to leave a link. :)