Just discovered that my N900 is trying to connect to an unknows wireless network because it was connected to one that uses the same name (SSID) eariler.
So if you connect to a wireless network called Linksys, it'll always connect to any network that has the same SSID, instead of verifying the MAC Address.

Can someone confirm this?
https://bugs.maemo.org/show_bug.cgi?id=6315

This is a feature. Think intra-ess roaming.

The weird thing is you leaving the default SSID.

I'm not, I was using my friend's wireless network.
Mine has WPA2, hidden SSID, MAC Address filtering

agogo, as javispedro said and Janne replied in the bug report, you cannot identify a wireless network by wlan. You must use SSID. Take me, for example: I'm walking around in this building with 10-20 wireless access points, all for the same network. My N800 is always using the one nearby, and seamlessly moving over to another one when I move away. And wherever I am in the building I can connect to the network, whatever AP is near. And think maintenance, some of these are getting replaced now and then.

It's just not possible to use MAC to identify wireless networks, so that's why it has to use SSID.