This one!!
I get it ALL the %*&#ing time!!
Usually when I'm trying to send an (important) email, or copying some (..important) files onto my ibook ready to run out the door and go to uni. When it appears, it usually pops up every few minutes in clusters, until I restart or stop doing whatever I was trying to do and let the ibook sleep on it.
Most often I get it when connecting to SMB/CIFS shares from my PC on my ibook. My wireless network uses two 802.11b/g ultraWAP APs; one in AP mode, and one in Universal Repeater mode (other end of the house), with an emac connected to it's ethernet port. I use WPA-PSK (TKIP) encryption, with a fairly secure password, so I'm sure nobody is haxxing me. Besides, it almost always occurs when I'm doing something funky with the network.
So, SO annyoying. A mysterious box that tells you basically nothing, and gives you no options? How very un-apple.
The picture above isn't from my system (which is 10.4.9) because I couldn't do a screengrab while that dialogue was open.
Thanks for any input,
Jeremy