The NM applet should have a tooltip off the tray icon displaying information about the network the system is connected to. in the netapplet code, netapplet_set_icon() function has this pretty much right. http://cvs.gnome.org/viewcvs/netapplet/src/netapplet.c?rev=1.6&view=auto
Fixed in CVS. Feel free to clean up the actual tip text and commit if you'd like.