I have 2 macs and often screen share. It works once then disconnects after a while and we cannot reconnect until we restart one of the laptops and then even that doesn't always work. We are on wifi on a router we have no control over. I noticed that we cannot ping each other once we are disconnected yet the WAN access works fine on both machines. One is snow leopard and the other lion. I did notice the bonjour browser on lion saying something like "can't resolve link-local name" yet funny enough it's the other laptop that needs to restart. Screen sharing works just fine over firewire. It's unclear whether all wifi routers do this.